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CONTINUING EVOLUTION OF DIAMIDO-SUPPORTED MOLYBDENUM IMIDO 

ORGANOMETALLIC CHEMISTRY 



By 
THOMAS M. CAMERON 



A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL 

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT 

OF THE REQUIREMENTS FOR THE DEGREE OF 

DOCTOR OF PHILOSOPHY 

UNIVERSITY OF FLORIDA 

2002 















TO LUCILE, MELISSA, AND GERRY 
























ACKNOWLEDGMENTS 
My undergraduate and Los Alamos experiences, shaped by Professor S. A. 
Westcott and Dr. R. Tom Baker, respectively, were certainly memorable and will never 
be forgotten. The skills I learned while working with Wesctcott and Baker have served 
me throughout graduate school. The time and effort they put forth guiding and helping 
me is evident in the work outlined in this dissertation, and were it not for these two 
individuals, I would never have continued my education in chemistry on the graduate 
level. 

Los Alamos is where I first met Professor James M. Boncella, who was visiting on 
sabbatical at the time. Based on our interaction, I decided to work for him as a graduate 
student and followed him back to UF. It was an excellent decision. Professor Boncella, 
with his never-ending knowledge of organometallic chemistry, has been responsible for 
my growth as a chemist over the last five years. Together we have made some interesting 
chemical discoveries and have published these results in leading journals in our field. I 
have enjoyed the time spent with him. Perhaps our paths will cross again in the future. 

Dr. Khalil Abboud has made great contributions to this dissertation. He has solved 
all of the structures reported herein, and it was always a pleasure to work with him. 
Without his hard work there would be, without a doubt, fewer structural studies in this 
report. 

Our work would not have been complete without the excellent support of Dr. Ion 
Ghiviriga. Over the past years Ion has taught me a great deal about 2-D NMR and has 

iii 



contributed to our work by characterizing several organometallic species. I will never 
forget the seven-course dinners at Ion's place, his generosity, and his hospitality. I just 
wish there were time to learn more from him. I thank Professor Michael Scott for all of 
his help over the years and for access to his automatic coffee machine. I will always 
remember the time spent with Tim Foley. He is a remarkable individual and things will 
not be the same without him. Thanks also go to Jeff for happy drinking days and Bob 
Shelton for setting up all of the DFT calculations mentioned in this work. 

I could not have accomplished any of this work without my loving parents, Lucile 
and Gerry. They have supported me unwaveringly for the last 27 years, never asking for 
anything in return. Had it not been for their encouragement, I could never have lasted. I 
do not know what I would have done without them. I love them both with all of my 
heart. 

Had I not made this journey to and through graduate school, I would never have 
met Melissa. She alone has made this experience worthwhile and has given my life a 
new meaning. Her strength, love, and support have helped me through difficult times. It 
has often seemed like the world was trying to keep us apart, but I believe that destiny 
brought us together, and together we will remain. I have missed her dearly since her 
graduation, and it is as if a part of me left with her. But we will soon be together again. 



IV 



TABLE OF CONTENTS 

page 

ACKNOWLEDGMENTS iii 

LIST OF TABLES i x 

LIST OF FIGURES x 

ABSTRACT x j v 

CHAPTER 

1 METAL IMIDES AND AMIDES 1 

Nitrogen Donor-Based Ligands in Early Metal Chemistry 1 

Imido Ligands 2 

Amido Ligands 6 

Reactive Imido-Diamido Complexes 7 

Group 4 imido-diamido complexes 9 

Group 6 imido-diamido complexes 10 

Scope of the Dissertation 14 

2 SYNTHESIS OF LEWIS BASE-STABILIZED MOLYBDENUM COMPLEXES: 
SOURCES OF REACTIVE MOLYBDENUM(IV) 16 

Generation of Imido-Diamido Pyridine Complexes 16 

Synthesis and Characterization of [Mo(NPh)(Py) 2 (o-(Me3SiN)2C 6 H4)] (47) 16 

Metal (t vs. <T Electronic Configuation and Diamido Ligand Folding 20 

Synthesis and Characterization of [Mo(NPh)rrara(Py) 2 (CO)(o-(Me3SiN)2C 6 H4)] 

(48) 24 

C-N Activation of an Amido Ligand: Synthesis of Imido-Bridged 49 from 47.... 30 

Generation of Imido-Diamido P(OMe) 3 Adducts 31 

Synthesis and Characterization of [Mo(NPh)(P(OMe)3)3(o-(Me3SiN)2C 6 H4)] 

(50) 31 

Synthesis and Characterization of [Mo(NPh)(P(OMe) 3 )2CO(o-(Me 3 SiN)2C 6 H4)] 

(51) 33 

Generation of Imido-Diamido PMe 3 Adducts 35 

Synthesis and Characterization of [Mo(NPh)(PMe 3 )3(o-(Me 3 SiN)2C 6 H4)] (52) ... 35 
Synthesis and Characterization of [Mo(NPh)(PMe 3 ) 2 CO(o-(Me 3 SiN)2C 6 H4)l 

(54) 36 

Generation of Imido-Diamido PMe 2 Ph Adducts .41 



Synthesis and Characterization of [Mo(NPh)(PMe 2 Ph)2(o-(Me3SiN)2C 6 H4)] 

(55) 41 

Synthesis and Characterization of [Mo(NPh)(PMe2Ph)2CO(o-(Me3SiN)2C 6 H4)] 

(56) 43 

Synthesis and Characterization of [Mo(NPh)(DMPE)(0-(Me 3 SiN) 2 C 6 H4)-u- 

(DMPE)Mo(NPh)(DMPE)(o-(Me 3 SiN)2C 6 H4)](57) 44 

Generation of an Imido-Diamido Arduengo Carbene (Imidazol-2-Ylidene) Adduct .. 46 
Summary 51 

3 SYNTHESIS AND REACTIVITY OF A MOLYBDENUM IMIDO-DIAMIDO 
STRETCHED DIHYDROGEN COMPLEX 53 

Characterization of Dihydrogen Complexes 54 

Solution NMR Spectroscopy: d HH and 'j H -D 55 

Solution NMR Spectroscopy: d m and NMR Relaxation Time (T{) 55 

Molybdenum Imido-Diamido Stretched Dihydrogen Complexes 56 

Characterization of [Mo(NPh)(PMe 3 )2(H2)(o-(Me 3 SiN)2C 6 H4)] (60) 57 

Characterization of [Mo(NPh)(PMe2Ph) 2 (H2)(o-(Me 3 SiN)2C6H4)] (66) 63 

Bonding in Molybdenum Imido-Diamido Stretched Dihydrogen Complexes 65 

Reaction of 50 and 57 with Dihydrogen Gas 66 

Reaction of 52 with Phenylsilane and Diphenylsilane 68 

Summary 68 

4 SYNTHESIS, CHARACTERIZATION, AND REACTIVITY OF A 
MOLYBDENUM (IV), r| 4 -BUTADIENE COMPLEX AND r) 2 -ALKYNE 
COMPLEXES 71 

Early Transition Metal Butadiene Complexes 71 

Molybdenum Imido-Diamido Butadiene Complexes 73 

Synthesis and Characterization of r| 4 -Butadiene Complex 

[(Mo(NPh)-n 4 -(H2C=CHCH=CH 2 )(o-(Me 3 SiN)2C6H4)] (73) 74 

Reactivity of [(Mo(NPh)-ri 4 -(H2C=CHCH=CH2)(o-(Me 3 SiN)2C 6 H4)] (73) with 

2-Butyne 7g 

Reactivity of [(Mo(NPh)-r) 4 -(H2C=CHCH=CH2)(o-(Me 3 SiN) 2 C 6 H 4 )] (73) with 
Acetone: Formation of [(Mo(NPh)(CH 2 CH=CHCH 2 C(Me) 2 0) 
(o-(Me 3 SiN) 2 C 6 H 4 )] (77) 84 

Synthesis and Characterization of n 4 -MVK Complex [(Mo(NPh)-r| 4 - 

(0=C(Me)CH=CH2)(o-(Me 3 SiN)2C 6 H 4 )] (78) 85 

Summary gg 

Early Transition Metal Alkyne Complexes 88 

Synthesis and Characterization of Molybdenum Imido-Diamido Alkyne 

Complexes [(Mo(NPh)-Ti 2 -(RCCR)(o-(Me 3 SiN) 2 C 6 H 4 )] (R = Me (75), 
Ph (79), SiMe 3 (80)) 89 

Reaction of [(Mo(NPh)-Ti 2 -(PhCCPh)(o-(Me 3 SiN) 2 C 6 H4)] (79) with tert-Butyl 

Isocyanide 91 

Summary 96 



VI 



5 REACTIVITY OF MOLYBDENUM OLEFIN AND ARENE COMPLEXES 

WITH UNSATURATED SUBSTRATES 98 

Reaction of Molybdenum Olefin Complexes with Imines: The Synthesis of 

Molybdenum Imido-Diamido r| 2 -Imine Complexes 98 

Reaction of Molybdenum Olefin Complexes with Acetone and Aldehydes: The 

Synthesis of Oxametallacyclopentanes 105 

Reactivity of Arene Complexes with Acetone 109 

6 EXPERIMENTAL DATA 112 

General Methods 112 

Synthesis and Characterization 113 

[Mo(NPh)(Py) 2 ( -(Me 3 SiN) 2 C 6 H 4 )](47) "ZZZll3 

[Mo(NPh)/rara(Py) 2 (CO)(o-(Me 3 SiN) 2 C 6 H 4 )] (48) 1 14 

Imido-Bridged, Bimetallic 49 1 14 

[Mo(NPh)(P(OMe)3)3(o-(Me 3 SiN) 2 C 6 H4)](50) Z""ll5 

[Mo(NPh)(P(OMe) 3 ) 2 CO(o-(Me 3 SiN) 2 C 6 H 4 )](51) 115 

[Mo(NPh)(PMe 3 ) 3 (o-(Me 3 SiN) 2 C 6 H 4 )](52) 115 

[Mo(NPh)(PMe 3 ) 3 (o-(Me 3 SiN)(NH)C 6 H 4 )] (53) 1 16 

[Mo(NPh)(PMe 3 ) 2 CO(o-(Me 3 SiN) 2 C 6 H 4 )](54) 116 

[Mo(NPh)(PMe 2 Ph) 2 (o-(Me 3 SiN) 2 C 6 H 4 )](55) 116 

[Mo(NPh)(PMe 2 Ph) 2 CO(o-(Me 3 SiN) 2 C 6 H4)] (56) 117 

[Mo(NPh)(DMPE)(o-(Me 3 SiN) 2 C 6 H 4 )-n-(DMPE)Mo(NPh)(DMPE) 

(o-(Me 3 SiN) 2 C 6 H 4 )] (57) 117 

[Mo(NPh)IMes(o-(Me 3 SiN) 2 C 6 H 4 )](59) 118 

[Mo(NPh)(PMe 3 ) 2 (H 2 )(o-(Me 3 SiN) 2 C 6 H4)] (60) 118 

[Mo(NPh)(PMe 2 Ph) 2 (H 2 )(o-(Me 3 SiN) 2 C 6 H 4 )] (66) 1 19 

[Mo(NPh)(PMe 2 Ph) 2 (o-(Me 3 SiN)(NH)C 6 H 4 )] (67) 1 19 

[Mo(NPh)(P(OMe 3 )) 3 (o-(Me 3 SiN)(NH)C 6 H4)] (68) 119 

Synthesis and Characterization of 71 119 

[(Mo(NPh)-r) 4 -(H 2 C-CHCH=CH 2 )(o-(Me 3 SiN) 2 C 6 H4)] (73) 120 

[(Mo(NPh)-n 4 -(2,3-dimethyl-l,3-cyclohexadiene)(o-(Me 3 SiN) 2 C 6 H 4 )] (74) 121 

[(Mo(NPh)-ri 4 -(2,3-dimethyl-l,3-cyclohexadiene)(o-(Me 3 SiN) 2 C6H4)](m5/7u) 

(74) 121 

Intermediates [jy«-(Mo(NPh)(C(Me)=C(Me)CH 2 CHCHCH 2 ) 

(o-(Me 3 SiN) 2 C 6 H 4 )] (76a) and [awri-(Mo(NPh)(C(Me)=C(Me) 

CH 2 CHCHCH 2 )(o-(Me 3 SiN) 2 C 6 H 4 )] (76b) 121 

[(Mo(NPh)(CH 2 CH=CHCH 2 C(Me) 2 0)(o-(Me 3 SiN) 2 C 6 H4)](77) 122 

[(Mo(NPh)-T 1 4 -(0=C(Me)CH=CH 2 )(o-(Me 3 SiN) 2 C 6 H 4 )](78) 122 

[(Mo(NPh)-n 2 -(MeCCMe)(o-(Me 3 SiN) 2 C 6 H 4 )](75) 122 

[(Mo(NPh)-r) 2 -(PhCCPh)(o-(Me 3 SiN) 2 C6H 4 )](79) 123 

[(Mo(NPh)-r) 2 -(Me 3 SiCCSiMe 3 )(o-(Me 3 SiN) 2 C 6 H 4 )] (80) 123 

Reaction of [(Mo(NPh)-r) 2 -(PhCCPh)(o-(Me 3 SiN) 2 C 6 H 4 )] (79) with tert-Butyl 

Isocyanide: Synthesis and Characterization of 82 123 



vn 



[Mo(NPh)-r| 2 -PhN=C(H)Ar(o-(Me3SiN)2C 6 H4)] (Ar = C 6 H 4 -;?-OMe) (83) 124 

[Mo(NPh)-ri 2 -PhN=C(Me)Ph(o-(Me 3 SiN)2C6H4)] (84) 124 

[Mo(NPh)EtNC(H)ArC(H)ArNEt(o-(Me 3 SiN) 2 C 6 H4)] (Ar - C 6 H 4 -/?-OMe) 

(85) 125 

[Mo(NPh)BzNC(H)ArC(H)ArNBz(o-(Me 3 SiN) 2 C 6 H 4 )] (Ar = C 6 H 4 - j p-OMe) 

(86) 125 

[(Mo(NPh)(C(Me)2CH 2 C(Me) 2 0)(o-(Me 3 SiN)2C6H 4 )] (88) 126 

[(Mo(NPh)(C(H)PhCH 2 C(Me) 2 0)(o-(Me 3 SiN)2C 6 H 4 )](89) 126 

[(Mo(NPh)(C(H)PhCH 2 C(Et) 2 0)(o-(Me 3 SiN)2C 6 H 4 )] (90) 127 

[(Mo(NPh)(C(H)PhCH 2 C(CH2)50)(o-(Me 3 SiN)2C6H 4 )] (91) 127 

[(Mo(NPh)(C(Me)2CH2C(H)(C 6 H 4 - j p-OMe)0)(o-(Me 3 SiN)2C 6 H 4 )] (92) 128 

Reactivity of Arene Complexes with Acetone 128 

LIST OF REFERENCES 129 

BIOGRAPHICAL SKETCH 139 



vm 






LIST OF TABLES 



Table page 









2-1 X-ray data for crystal structures 19, 47, and 48 19 

2-2 X-ray data for crystal structures 49, 50, and 51 38 

2-3 X-ray data for crystal structures 55, 57, and 59 50 

4-1 X-ray data for crystal structures 73, 74, and 78 77 

4-2 X-ray data for crystal structures 75 and 80 94 

5-1 X-ray data for crystal structures 84, 85, and 88 100 



IX 



LIST OF FIGURES 
Figure page 

1-1 Metal-imido multiple-bond interactions 2 

1-2 General valence bond description of possible metal-imido interactions 3 

1-3 Representation of a generic metal (dn)-im\do (pii) multiple-bonding interaction... 4 

1-4 Mo(NPh)2(S 2 CNEt 2 )2 (1) 4 

1-5 Cp* 2 Ta(NPh)H (2) 5 

1-6 Current multidentate amido ligands in organometallic chemistry 9 

1-7 Synthesis of Group 4 imido-diamido complexes 10 

1-8 Reactivity of Group 4 imido-diamido complexes 11 

1-9 Molybdenum (17) and tungsten (18) imido-diamido dichlorides 11 

1-10 Synthesis of molybdenum and tungsten dialkyl complexes 12 

1-1 1 Reactivity of tungsten dialkyls with Bu'NC 12 

1-12 Formation of molybdenum and tungsten alkylidene adducts 13 

1-13 Formation and reactivity of metallacycle 36 13 

1-14 Molybdenum olefin complexes and the synthesis of arene complexes 14 

2-1 Synthesis of [Mo(NPhXPy) 2 (o-(Me 3 SiN) 2 C 6 H 4 )] (47) 17 

2-2 Enlarged region of the 'H NMR spectrum of 47 at -20°C 17 

2-3 Thermal ellipsoid plot of 47 (50% probability thermal ellipsoids) 18 

2-4 Space-filling model of 47. Steric crowding hinders pyridine ligand rotation 20 

2-5 Thermal ellipsoid plot of 19 (50% probability thermal ellipsoids) 21 

2-6 Diamido ligand folding in molybdenum imido-diamido complexes 23 






\ 



2-7 Ligand-folding and a general 3 orbital 4e" interaction 23 

2-8 Model systems for DFT studies on 19 and 47 24 

2-9 Optimized geometries for 19a, 19b, and 19c, emphasizing ligand folding 25 

2-10 Important MO interactions for 19c 26 

2-1 1 Important MO interaction for 47c 27 

2- 1 2 Synthesis of [(Mo(NPh)/rfl/w(Py) 2 (CO)(o-(Me 3 SiN)2C6H4)] (48), 

Py = pyridine 28 

2-13 Thermal ellipsoid plot of 48 (50% probability thermal ellipsoids) 29 

2-14 C-N activation of the o-(Me 3 SiN) 2 C 6 H4 ligand in 47 31 

2-15 Thermal ellipsoid plot of 49 (50% probability thermal ellipsoids) 32 

2-16 Synthesis of [Mo(NPh)(P(OMe) 3 ) 3 (o-(Me 3 SiN) 2 C 6 H 4 )] (50) 32 

2-17 Thermal ellipsoid plot of 50 (50% probability thermal ellipsoids) 34 

2-18 Thermal ellipsoid plot of 51 (50% probability thermal ellipsoids) 37 

2-19 Thermal ellipsoid plot of 52 (50% probability thermal ellipsoids) 39 

2-20 Thermal ellipsoid plot of 53 (50% probability thermal ellipsoids) 40 

2-21 Synthesis of [(Mo(NPh)(PMe 3 )2(CO)(0-(Me 3 SiN) 2 C 6 H 4 )] (54) 41 

2-22 Synthesis of [Mo(NPh)(PMe 2 Ph) 2 (o-(Me 3 SiN)2C 6 H4)] (55) 42 

2-23 Thermal ellipsoid plot of 55 (50% probability thermal ellipsoids) 43 

2-24 Synthesis of [Mo(NPh)(PMe 2 Ph) 2 CO(o-(Me 3 SiN) 2 C 6 H 4 )] (56) 44 

2-25 Synthesis of [Mo(NPh)(DMPE)(o-(Me 3 SiN) 2 C 6 H4)-u-(DMPE) 

Mo(NPh)(DMPE)(o-(Me 3 SiN) 2 C 6 H 4 )](57) 45 

2-26 Thermal ellipsoid plot of 57 (50% probability thermal ellipsoids) 46 

2-27 Synthesis of [Mo(NPh)IMes(o-(Me 3 SiN) 2 C 6 H 4 )] (59) 48 

2-28 Thermal ellipsoid plot of 59 (50% probability thermal ellipsoids) 49 

2-29 Carbon monoxide complexes and vCO (cm" 1 ) 52 

3-1 General bonding in H 2 complexes 54 

xi 
. . 



3-2 Generation and reactivity of H2 complex 60 57 

3-3 No H/D exchange between N-D and Si-H sites at 20°C 58 

3-4 Spectra ('H NMR) of the H 2 and H-D ligands of 60 and 60D (-20°C) 59 

3-5 Relaxation time vs. temperature plot for 60, In T\ vs. K" 1 60 

3-6 Potential mechanism for the formation of 53 61 

3-7 Potential mechanism for the formation of 53 62 

3-8 H and D do not partition equally between N and Si sites 62 

3-9 Generation and reactivity of H2 complex 66 64 

3-10 Spectra ('H NMR) of the H 2 and H-D ligands of 66 and 66D (-20°C) 64 

3-1 1 Relaxation time vs. temperature plot for 66, In T\ vs. K" 1 65 

3-12 Bonding scenario for H 2 complexes 60 and 66 66 

3-13 Reaction of 50 and 57 with Dihydrogen Gas 67 

3-14 Possible H 2 complex in the DMPE system 67 

3-15 Generation of cyclic 71 and 72 69 

4-1 Synthesis of zirconocene butadiene complexes 72 

4-2 Possible tc and a , n structures for c/s-butadiene complexes 72 

4-3 Reactivity of Cp 2 Zr(butadiene) with a representative unsaturated substrate 73 

4-4 Synthesis of [(Mo(NPh)-n 4 -(H 2 C=CHCH=CH 2 )(o-(Me 3 SiN) 2 C 6 H4)] (73) 74 

4-5 Thermal ellipsoid plot of 73 (50% probability thermal ellipsoids) 76 

4-6 Reactivity of [(Mo(NPh)-ri 4 -(H 2 C=CHCH=CH 2 )(o-(Me3SiN) 2 C6H4)] (73) with 

2-butyne 79 

4-7 Thermal ellipsoid plot of 74 (50% probability thermal ellipsoids) 80 

4-8 Structure of intermediate (76a), showing selected carbon (underlined) and 

proton chemical shifts, assigned by NMR spectroscopy 82 

4-9 Structure of intermediate (76b), showing selected proton chemical shifts, 

assigned by NMR spectroscopy 82 



xn 



4-10 Proposed mechanism for reaction of 73 with 2-butyne 83 

4-1 1 Formation of [(Mo(NPh)(CH2CH=CHCH2C(Me)20)(o-(Me 3 SiN)2C6H4)] (77)...84 

4-12 Proposed structure for 77 showing selected carbon (underlined) and proton 

chemical shifts, assigned by NMR spectroscopy 84 

4-13 Possible methyl group exchange pathway 86 

4-14 Thermal ellipsoid plot of 78 (50% probability thermal ellipsoids) 87 

4-15 Bonding in transition metal alkyne complexes 89 

4-16 Synthesis of [(Mo(NPh)-ti 2 -(RCCR)(o-(Me3SiN)2C 6 H4)] (R - Me (75), Ph (79), 

SiMe 3 (80)) 90 

4-17 Thermal ellipsoid plot of 75 (50% probability thermal ellipsoids) 92 

4-18 Thermal ellipsoid plot of 80 (50% probability thermal ellipsoids) 93 

4-19 Reaction of [(Mo(NPh)-r) 2 -(PhCCPh)(o-(Me 3 SiN) 2 C 6 H4)] (79) with Bu'NC: 

formation of 82 95 

4-20 Proposed structure of 82 showing selected carbon (underlined) and proton 

chemical shifts, assigned by NMR spectroscopy 95 

4-21 Proposed mechanism for the formation of 82 97 

5-1 Synthesis of r) 2 -imine complexes 83 and 84 99 

5-2 Thermal ellipsoid plot of 84 (40% probability thermal ellipsoids) 101 

5-3 Reductive coupling of imines 103 

5-4 Thermal ellipsoid plot of 85 (50% probability thermal ellipsoids) 104 

5-5 General scheme for the reductive coupling of organic molecules 1 06 

5-6 Synthesis of oxametallacyclopentanes 106 

5-7 Thermal ellipsoid plot of 88 (30% probability thermal ellipsoids) 108 

5-8 Structural assignment of 89 including assigned proton chemical shifts 109 

5-9 Reaction of arene complexes with acetone Ill 

5-10 Possible equilibrium between an arene complex and a metallated species Ill 



xin 



Abstract of Dissertation Presented to the Graduate School 

of the University of Florida in Partial Fulfillment of the 

Requirements for the Degree of Doctor of Philosophy 

CONTINUING EVOLUTION OF DIAMIDO-SUPPORTED MOLYBDENUM IMIDO 

ORGANOMETALLIC CHEMISTRY 

By 

Thomas M. Cameron 
December 2002 

Chair: James M. Boncella 
Department: Chemistry 

We explore the synthesis and reactivity of diamido-supported molybdenum imido 

complexes based on the parent complex [Mo(NPh)Cl2(o-(Me 3 SiN) 2 C 6 H 4 )]. Reaction of 

olefin complexes [Mo(NPh)(propene)(o-(Me 3 SiN)2C 6 H4)] (37) or [Mo(NPh)(isobutylene) 

(o-(Me 3 SiN) 2 C 6 H4)] (38) with Lewis bases Py (pyridine), P(OMe) 3 , PMe 2 Ph, DMPE, and 

IMes (l,3-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) gives the corresponding adducts 

[Mo(NPh)(Py)2(o-(Me 3 SiN) 2 C 6 H4)] (47), [Mo(NPh)(P(OMe) 3 ) 3 (o-(Me 3 SiN) 2 C 6 H 4 )] (50), 

[Mo(NPh)(PMe 2 Ph)2(o-(Me 3 SiN)2C6H4)](55), [Mo(NPh)(DMPE)(o-(Me 3 SiN)2C 6 H4)-n- 
(DMPE)Mo(NPh)(DMPE)(o-(Me 3 SiN) 2 C 6 H 4 )] (57), and [Mo(NPh)IMes 
(o-(Me 3 SiN) 2 C 6 H 4 )] (59), respectively. In this study, we report the X-ray structures of 
47, 50, 55, 57, and 59. We also discuss the reactivity of these complexes, as well as the 
PMe 3 adduct [Mo(NPh)(PMe 3 ) 3 (o-(Me 3 SiN) 2 C 6 H 4 )] (52), with CO gas, generating 
[(Mo(NPhHram-(Py) 2 (CO)(o-(Me 3 SiN) 2 C 6 H 4 )](48), [Mo(NPh)(P(OMe) 3 ) 2 CO 
(o-(Me 3 SiN) 2 C 6 H 4 )] (51), [(Mo(NPh)(PMe 3 ) 2 (CO)(o-(Me 3 SiN) 2 C 6 H 4 )] (54), and 

xiv 



[Mo(NPh)(PMe 2 Ph)2CO(o-(Me3SiN)2C 6 H 4 )] (56). We report the X-ray structures of 48 
and 51. We also present a DFT study comparing the bonding in complexes 47 and 
dialkyl complex [Mo(NPh)(Me)2(o-(Me 3 SiN)2C 6 H4)] (19). These results indicate that the 
degree of ligand folding in complexes related to 19 may be influenced by steric factors. 
We report the X-ray structure of 19. The unusual C-N activation reactivity of 47 is also 
discussed. 

Dihydrogen complexes [Mo(NPh)(PMe3)2(H2)(o-(Me3SiN)2C 6 H4)] (60) and 
[Mo(NPh)(PMe 2 Ph)2(H2)(o-(Me 3 SiN) 2 C 6 H4)] (66) are formed by treatment of 52 and 55 
with H 2 gas, respectively. The d m of compounds 60 and 66, as determined by T l{m m) and 
Jh-d, are in good agreement and indicate that the H2 ligand is slowly rotating. The 
reactions of 50 and 57 with H2 gas are also presented. 

Complexes 37 and 38 react with butadiene gas, generating [(Mo(NPh) 
n 4 -(H2C=CHCH=CH2)(o-(Me 3 SiN)2C 6 H4)] (73). We report the solid-state structure of 
73, showing that 73 is best described as a rc 2 -butadiene complex, and we discuss the 
reactivity of 73 with 1.0 and 2.0 equiv of 2-butyne. Reaction of 73 with one equivalent 
of 2-butyne gives a molybdenum 2,3-dimethyl-l,3-cyclohexadiene complex (74). The 
X-ray structure of 74 is reported. Treatment of 73 with 2.0 equiv of 2-butyne gives 
[(Mo(NPh)-n 2 -(MeCCMe)(o-(Me3SiN)2C 6 H4)] (75) and l,2-dimethyl-l,4- 
cyclohexadiene as products. Complex 75 as well as [(Mo(NPh)r| 2 -(PhCCPh) 
(o-(Me 3 SiN) 2 C 6 H4)] (79) and [(Mo(NPh)-T 1 2 -(Me 3 SiCCSiMe3)(o-(Me 3 SiN)2C 6 H 4 )] (80) 
are synthesized independently and X-ray structures of 75 and 80 are presented. The allyl 
metallacycles[^«-(Mo(NPh)(C(Me)=C(Me)CH2CHCHCH2)(o-(Me3SiN)2C 6 H4)](76a) 









xv 



and [a«r/-(Mo(NPh)(C(Me)=C(Me)CH 2 CHCHCH2)(o-(Me3SiN) 2 C 6 H4)] (76b) are 
identified as intermediates in the reaction of 73 with 2-butyne by NMR spectroscopy. 

Reaction of 38 with aldimines and ketimines gives r| 2 -imine complexes 
[Mo(NPh)-Ti 2 -PhN=C(H)Ar(o-(Me 3 SiN)2C 6 H 4 )] (Ar = C 6 H 4 -/?-OMe) (83) and 
[Mo(NPh)-Ti 2 -PhN=C(Me)Ph(o-(Me 3 SiN) 2 C 6 H 4 )] (84). The X-ray structure of 84 is 
reported. For less sterically demanding aldimines, the reductive coupling products 
[Mo(NPh)EtNC(H)ArC(H)ArNEt(o-(Me 3 SiN) 2 C 6 H 4 )] (Ar - C 6 H 4 -/?-OMe) (85) and 
[Mo(NPh)BzNC(H)ArC(H)ArNBz(o-(Me 3 SiN) 2 C 6 H 4 )] (Ar = C 6 H 4 -/>OMe) (86) are 
isolated, and the X-ray structure of 85 is reported. Olefin complexes react with ketones 
and aldehydes, giving compounds [(Mo(NPh)(C(Me) 2 CH 2 C(Me) 2 0)(o-(Me 3 SiN) 2 C 6 H 4 )] 
(88) and [(Mo(NPh)(C(H)PhCH 2 C(Me) 2 0)(o-(Me 3 SiN) 2 C 6 H 4 )] (89). The X-ray structure 
of 88 is reported. 






xvi 






CHAPTER 1 
METAL IMIDES AND AMIDES 

Organometallic chemistry is ultimately concerned with bonding interactions 
between transition metals and organic fragments through a and/or 7i-bonds. ' Transition 
metal fragments often stabilize reactive organic molecules and participate in catalytic 
carbon-carbon, carbon-nitrogen, carbon-hydrogen, and various other bond-forming and 
bond-breaking reactions. As such, modern organometallic chemistry has become an area 
rich in diverse chemical structure as well as a versatile tool for the synthetic chemist in 
the 21 st century. 

The reactivity observed at a metal center is highly dependent on the ancillary 
ligands that support that particular metal species. Electronic effects, such as 7i-loading 
(more ligand 7i-donor orbitals than metal 7i-acceptor orbitals), as well as the steric 
requirement of an ancillary ligand set dictate reactivity at the metal-center. Thus, efforts 
toward the derivitization of existing ancillary ligands and the discovery of new ligands 
with the goal of "tuning" reactivity at a metal-center are a major part of organometallic 
chemistry. Recent developments using this theme involve applications of imido and 
diamido ligands in organometallic synthesis. 

Nitrogen Donor-Based Ligands in Early Metal Chemistry 

Nitrogen donor-based ligands are common to the coordination sphere of early 
metals in organometallic chemistry and are prevalent in the literature. Ligands 
representative of this class include amines, 1 r) 1 or r\ 2 nitriles, 2 ' 3 r| 2 -imino-acyl ligands, 4,5 
terminal or bridging nitrosyls, 6 terminal or bridging dinitrogen, 7 nitrides, 8 and 

1 






poly(pyrazolyl)borates. 9 It is beyond the scope of this introduction to review each of 
these ligand types individually, and the reader is referred to the leading literature. 
Organometallic complexes of r) 2 -imine ligands are also known and are discussed in 
Chapter 5. 
Imido Ligands 

The area of transition-metal imido chemistry has experienced rapid and sustained 
growth over the last 20 years, and several excellent review articles have been written on 
the subject. The first major review to treat the topic was put forth by Nugent and 
Haymore. 10 Wigley recently published an in-depth review," while Metal-Ligand 
Multiple Bonds by Nugent and Mayer provides an excellent overview of imido 
complexes. 12 An extensive review of this work is not possible here. The subsequent 
discussion is limited to structural and bonding considerations in imido complexes. 

The imido ligand is usually referred to as a closed-shell dianion, implying that the/? 
orbitals of the ligand are filled. From a general standpoint, the imido or NR" 2 ligand can 
coordinate to a metal center through a metal-nitrogen multiple bond involving one a and 
either one or two additional 7i-interactions. In this way, it is possible to have a 
metal-nitrogen double-bond (A; one a and one ^-interaction) or triple-bond (B; one a and 
two ^-interactions) interaction (Figure 1-1). 

,N=rM R— NEM 

A B 



R X 



Figure 1-1 . Metal-imido multiple-bond interactions. A) A metal-nitrogen double-bond 
interaction; B) A metal-nitrogen triple-bond interaction. R = an organic 
functionality such as alkyl or aryl. 



These bonding interactions can be described by a simple localized valence bond 
approach. In this treatment the hybridization at the imido nitrogen atom partially dictates 
the structural parameters of the imido ligand (Figure 1-2). In C the nitrogen atom is sp 2 
hybridized implying, from a first approximation, a considerably bent R-N-M linkage 
(125° to 140°) with the lone pair occupying an N(sp 2 ) hybrid orbital. 10 " 12 In such a 
situation, the imido ligand is formally considered a 2e" donor for electron-counting 
purposes (the neutral method of e" counting is used throughout this manuscript) and a 
formal metal-nitrogen double bond exists. Type D represents a linear structure that can 
arise when molecular orbital (MO) interactions do not allow for adoption of structure 
type C. The linear imide, represented by E, can come about when two ^-interactions are 
present, forming a metal-nitrogen triple bond. Imides of type E contribute 4e" to the 
electron count of a metal. For E the ^-interactions are best described, in MO terms, as 
overlap between filled, imido, nitrogen-based p x and p y atomic orbitals and empty metal 
d xz and d yz atomic orbitals, respectively, as shown for a general case in Figure 1-3. 



,N=M 
R 


t 

R— N= 




=M 


R- 


-N= 


=M 


N sp (2e" donor) 


N sp (2e" 


donor) 


Ns/ 


>(4e 


" donor) 


C 


D 






E 





Figure 1-2. General valence bond description of possible metal-imido interactions 

Few complexes of type C contain strongly bent-imido ligands. Of these 
compounds, the best known example is the molybdenum complex Mo(NPh) 2 (S 2 CNEt 2 )2 
(1) (Figure 1-4), a compound with one bent (type C; C-N-Mo = 139.4(4)°; N-Mo = 
1.789(4) A) and one linear (type E; C-N-Mo = 169.4(4)°; N-Mo - 1.754(4) A) imido 



ligand (the 169.4(4)° angle is not linear, however, and as discussed below, type E imido 
linkages can have angles between 150° and 180°). 13 With a bent-imido structure, 1 is 
formally considered an 1 8 e" complex with the bent imido contributing 2e" and the linear 
imido contributing 4e" to the electron count. It has been proposed that the lone-pair 
electron localization in C can be attributed to a pair of electrons residing in a 
nitrogen-based MO on the bent-imido nitrogen, which is nonbonding with respect to the 
Mo-N interaction in 1. 



cL 



Z 
i 




Y 



X 




"yz 



Figure 1-3. Representation of a generic metal (dn)-imido (pn) multiple-bonding 
interaction. F) interaction of an imido-based/?* with a metal-based d*, 
orbital; G) interaction of an imido-based/?,. with a metal-based d y: orbital. 













Figure 1-4. Mo(NPh) 2 (S 2 CNEt 2 )2 (1) 









Few examples of imido complexes conform to the limiting structure D in the 
literature. The Cp*2Ta(NPh)H complex (2) reported by Bercaw 14 may be the closest 
related compound having a very long (1.831(10) A) Ta-N bond and a linear Ta-N-C 
angle of 177.8(9)° (Figure 1-5). With a 2e" donor imido ligand, 2 is an 18e" complex, and 
the remaining imido electrons are not needed for bonding at the metal. The long Ta-N 
bond length, coupled with sp hybridization at the imido nitrogen (implied by the linearity 
of the imide), led most to conclude that the structure of 2 was best, although questionably 
defined in valence bond terms, as occupying a position halfway between D and E. 
Jorgensen put forth an accurate and elegant explanation of this bonding situation using 
extended Hiickel calculations to clear the matter up. Increasing the imido angle causes an 
increase in the antibonding nitrogen-carbon (imido nitrogen-ipso carbon) overlap 
population in the HOMO (highest occupied molecular orbital), which destabilizes the 
bent-imido molecule (see Jorgensen 15 for the full MO analysis). This example shows that 
this valence bond description can often be improved upon, in some cases by a substantial 
margin. 



X Ta"'H 




2 
Figure 1-5. Cp* 2 Ta(NPh)H (2) 

Most imido ligands encountered in organometallic and inorganic chemistry are 
those described by the limiting structure E, and most examples in this dissertation fall 
into this category. It is important to note that although E implies a linear R-N-M unit, in 
practice this angle can vary from approximately 1 50° to 1 80°. Experimental and 






theoretical studies of complexes with metal-nitrogen triple-bond interactions have shown 
a very soft bending potential associated with this angle. 16 It is therefore difficult to 
correlate the overall bond order to an R-N-M bond angle, and although such general 
approaches should be avoided, they still appear in the most current and respectable 
journals. 17 

In summary, while the valence bond treatment put forth is useful in gaining a basic 
understanding of metal-imido bonding, it is very limited. It seems that the best way to 
determine the true character of an imido complex is through a combination of reactivity, 
structural, and modern computational studies. 
Amido Ligands 

The widespread interest in amido ligands formulated as NH 2 \ NHR", or NR 2 " stems 
from their presence in a diverse array of compounds from the biologically significant, 
such as chlorophyll, to the area of inorganic chemistry, with the synthesis of 
amide-stabilized lanthanide metal centers. Putting aside the bioinorganic chemist's 
interest in the biologically significant amido-based ligands, initial exploration of 
metal-amides focused on structure and bonding as it compared to metal-carbon bonds of 
the time. This initial foray into metal-amido chemistry took place in the 1960s and early 
1970s, and excellent reviews on the subject have been written. 1819 Researchers realized 
that the metal-amido bond in early metal complexes was rather inert in comparison to the 
metal-carbon bond; and they lost interest in the topic to pursue more interesting and 
lucrative chemistry, such as the developing field of early metal, metallocene-mediated 
polymerization. 20 Research in the field slowed until the inertness of the amido ligand 
was used to stabilize reactive early metal centers. Ironically, one such discovery 
involved the application of chelating araa-monocyclopentadienyl-amido ligands to early 



metal-olefin polymerization. The ability of a chelating amide to stabilize an early metal 
center, coupled with the synthetic ease and diversity of ligand design, led to a 
resurrection of metal-multidentate amido chemistry in the 1990s that has continued into 
this century. 

Recent developments involving multidentate amido ligands have been 

90 99 

reviewed. ' A list of precursors to some of the most important multidentate amido 
ligands in organometallic chemistry today is shown in Figure 1-6. These examples may 
be used to point out properties that make multidentate amides so useful. The availability 
of two substituent positions on an amide allow for the incorporation of this functionality 
into podand (3) 23,24 -and macrocycle (4) 25 -like systems. They allow the chemist to exert 
steric and electronic control during ligand design (5, 22 6, 26 7 27 ) and allow the amide to be 
easily appended to other donor functionalities to better suit a metal center (8). 28 
Furthermore, several synthetic strategies can be used to introduce amides to the 
coordination sphere of a metal. 

These ligands are extremely useful in metal chemistry. For example, uranium 
species of trianionic 3 form dinitrogen complexes under appropriate conditions, 29 and 
Group 4 complexes of 5 and 6 are practical precursors to cationic olefin polymerization 
catalysts. 22,26 Metal complexes of 7, 8, and related ligands are discussed in the next 
section, which is devoted to imido-diamido chemistry. 
Reactive Imido-Diamido Complexes 

The metal-imido multiple bonds impart unique properties to the imide and metal 
fragment, creating extremely reactive or stable molecules depending on the identity of the 
metal, the oxidation state, the imido substituent, and other ancillary ligands. 30 
Multidentate amido ligands, when used as these ancillary ligands, should be key in tuning 



8 

reactivity at the metal center. It is thus reasonable to assume that one can create desired 
reactivity at an early metal center through judicious choice of both the imido and 
multidentate amido ligand. Some considerations must be taken into account here. For 
the purposes of this discussion, we will consider only diamido and imido ligands. An 
imide and a diamide contribute a total of +4 to the formal oxidation state of a metal 
complex. Most imido complexes are oft/ 1 , d\ and d 2 metal centers, and the metal-imido 
multiple bonding requires that the d orbitals involved in bonding be empty. Furthermore, 
the desired reactivity of the new compound must be considered; should the imide be 
reactive or should both the imide and multidentate amide behave as ancillary ligands? If 
Group 4 species are targeted, the metal in the resulting metal-imide-diamide, of the type 
M(NR) 2 (NR 2 )2, will be in a formal +4 oxidation state. Useful functionalization will not 
be possible unless achieved through reactivity of the imido functionality. Synthesis of 
similar Group 5 complexes will suffer from the same disadvantages. Using a Group 6 
metal in an analogous fashion could yield complexes of the type M(NR) 2 (NR2)2X 2 , 
allowing the metal to be functionalized at X. While high oxidation states and imido 
complexes are known for Groups 7 and 8, amides of these metals are rare. Appling the 
same idea to Group 9 or 10 metals will be difficult, for amides are not extremely good 
late-metal ligands, and most imido complexes must be of high-oxidation-state metal 
centers that are usually not available this late in the transition metal series. Based on this 
reasoning, efforts directed toward the synthesis of Group 6 imido-diamido complexes 
seem the most plausible. We have taken the above-mentioned approach for Group 6 
metals; others have explored the possibilities from Groups 4 through 6. The important 
results are presented here. 



Me 
,SiMe3 

Me 3 Sl ^NH HN 
HN 

-N 

3 



Ar 
N-SiMe 3 



N-SiMe 3 ^^NH 

Ar SiMe 




Figure 1 -6. Current multidentate amido ligands in organometallic chemistry 
Group 4 imido-diamido complexes 

In general, the chemistry of Group 4 imido complexes is driven by reactive imido 
functionalities. Recently Mountford and coworkers 30 have made great strides in 
nonmacrocycle-related developments in this area by focusing on the diamido ligands 5, 8, 
and 9 (Figures 1-6 and 1-7). The synthesis of relevant Group 4 metal complexes is 
shown in Figure 1-7. The reactivity of complex 13 with unsaturated organic compounds 
has been extensively studied and usually involves coupling reactions between the 
metal-imide and reactant. For example, 13 (M = Ti, R = butyl 1 ) reacted with 
1-phenylpropyne at 80°C to give 15 and with 2,6-diisopropylphenyl isocyanate to afford 
16 (Figure 1-8). 3 ' 



Similar chemistry has also been reported for titanium aryl or alkyl imido complexes 
and tetraaza macrocyclic diamido ligands similar to 4 (Figure 1-6). 25 ' 32 ' 33 ' 34 Reactivity of 
this nature at the imido functionality is indeed important and has most recently been 
implicated in the metathesis of imines 35 and the hydroamination of unsaturated organic 
substrates. 8 Perhaps the most remarkable imido reactivity has been observed by 



10 



39 



Wolczanski and coworkers, who reported C-H activation of saturated and unsaturated 
hydrocarbons by imido titanium species. 

R 

N SiMe 3 




Ti^/SiMe 3 




'N> 



Li 2 8 

R 

/ 

Py^ | ^Cl 
Py 
10 



Li 9 9 



NH NH 

Me,Si' X SiMe, 



Li,5 




SiMe, 



N 

,. A N ^.Ti=N-R 
Me 3 Si \f | 

Me 3 Si' p y 

12 



<y 




*6n/,J. x T „ Heat/10" 5/6 mbar 4L_ V 

Me 3 Si Py Me3Si - R 



13 



14 



Figure 1-7. Synthesis of Group 4 imido-diamido complexes. For 10: when M = Ti, R = 
butyl 1 or 2,6-C 6 H 3 Pr i 2 ; when M = Zr, R = 2,6-C 6 H 3 Pr i 2 . 

Group 6 imido-diamido complexes 

The imido-diamido complexes of the Group 6 metals do not generally have reactive 
imido functionalites. The imido and diamido ligands act as ancillary ligands and stabilize 
unusual complexes for these high-oxidation-state species. Mountford 30 has reported a 
few complexes of Group 6 imide-diamides with the ligand precursor 9. In contrast, 
Boncella has developed extensive chemistry in this area starting from the two metal 
dichloride species shown in Figure 1 -9. 












11 



N" 

Me 3 Si^ i=N " But 
Me 3 Si py 



^ 




N 



Me- 



-Ph VN/„ 

— *■ Me 3 Si' n 

Me 3 Si' N 



^Ti" 



-Ph 



13 



Bu l 
15 



H 



Pr 1 

^-NCO 
Pr 1 




Me 3 Si A N ^ 
Me,Si 



Figure 1-8. Reactivity of Group 4 imido-diamido complexes. Reactivity of 13 with 
1-phenylpropyne and 2,6-diisopropylphenyl isocyanate. 




Me 3 Si 

N^Mo^'Cl 





Me 3 Si 

N <-i 




17 18 

Figure 1 -9. Molybdenum (17) and tungsten (18) imido-diamido dichlorides 

Molybdenum and tungsten alkyl and alkylidene complexes. The dichlorides 17 
and 18 were easily converted to alkyl complexes upon treatment with 
non-p-hydrogen-containing alkyl magnesium reagents (Figure 1-10). 27 ' 40 The reactivity 
of tungsten alkyls 24 and 26 has been explored with ter/-butyl isocyanide (Bu'NC) 



12 



(Figure 1-1 1). 41 Complexes 24 and 26 inserted Bu'NC into each metal alkyl bond, 
affording the r) 2 -imino-acyl complexes 30 and 31. When heated, 30 underwent a 
carbon-carbon coupling reaction giving 32. This insertion and coupling chemistry has 
been well-documented for early metals with CO or isocyanides. 4 ' 5 ' 41 Similar chemistry 
has not been seriously explored with the molybdenum alkyls 19-23. 






Ph 



17orl8 + 2.0RMgX 



Et,0 



\ 
N 



-78 °C— R.T. Me 3 Si^ 



Me 3 Si 

N' M^'R +2.0XC1 













^ 



R 



M - Mo: R = Me (19), Ph (20), CH 2 CMe 3 (21), CH 2 Ph (22), CH 2 SiMe 3 
(23); M = W: R = Me (24), Ph (25), CH 2 CMe 3 (26), CH 2 Ph (27), CH 2 SiMe 3 
(28), CH 2 CMe 2 Ph (29); X = halide. 

Figure 1-10. Synthesis of molybdenum and tungsten dialkyl complexes 




24 or 26 



Bu ' NC Me,Si V 



N 



R.T 



Me.Si 



N> W 




Toluene Me Si 

~* n S:""N 



Bu 



V (only 30) A/ N M 



N (only 30) 

w 



Me 



32 



R = Me (30), CH 2 CMe 3 (31) 

Figure 1-11. Reactivity of tungsten dialkyls with Bu'NC 

Treating complexes 21 and 26 with excess PMe 3 resulted in the formation of PMe 3 
alkylidene adducts 33 and 34 through an a-abstraction process (Figure 1-12). Attempts 
to generate the base-free tungsten analogue of 34 (35) by thermolysis of 26 gave rise to 
36, mainly via metalation of the diamido ligand of 35 (Figure 1-13). When 36 was 



13 



treated with PMe 3 , 34 was produced by trapping 35 with phosphine. Small amounts of 
36 were also formed via y-abstraction of the diamido ligand of 26 (see Vaughan et. al. 42 
for a more detailed mechanistic analysis). Reactivity of the molybdenum alkylidene (33) 

27 

remains largely unexplored. 

Bu l 



H 



Me.Si 



tt k PMe 3, N- M^ 

"^rT Me3Si 7H/ 



,xPMe, 





+ CMe 4 



M = Mo (33), W (34) 
Figure 1-12. Formation of molybdenum and tungsten alkylidene adducts 



26 



70 °C 



Toluene 



H. 



Bu l 



Me 3 Si 

N W^ XT 

Me.Si^ </ ^ N 

N 



35 





H Bu l 

H-V 
Me 3 Si | ^ n 
N % 





Figure 1-13. Formation and reactivity of metallacycle 36 

The (3-hydrogen-containing dialkyl complexes of tungsten are stable and have been 
synthesized in a fashion analogous to 24-29. 43 In general, coordinatively unsaturated 
p-hydrogen-containing complexes decompose by p-abstraction or elimination. The 
stability of these alkyl complexes is therefore surprising. Addition of PMe 3 to some of 
these species promoted p-abstraction and the formation of olefin complexes. This 
behavior is clearly unexpected, as the dissociation of ligands to yield unsaturated species 
usually induces P-abstraction. 43, 44 ' 45 



14 



Molybdenum and tungsten arene and molybdenum olefin complexes. Unlike 
the tungsten analogues, (3-hydrogen-containing molybdenum dialkyl complexes 
decompose readily at 20°C (room temperature) to yield olefin complexes. 46 These 
high-oxidation-state olefin complexes are rare and were hydrogenated in the presence of 
arenes, forming high-oxidation-state arene complexes (Figure 1-14). 47 High valent 
Group 6 arene complexes, with the exception of the tungsten complex mentioned here, 
are nonexistent; and generation of arene complexes in this way remains a rare process. 

We recently synthesized related tungsten arene complexes that showed interesting 
reactivity with phenylacetylene. 48 Furthermore, several bis(pyridine) complexes 49 were 
generated from this arene complex and were used to activate the carbon-sulfur bond of 



thiophenes 



50 



Ph 



Ph 



N 



R, 



Me 3 Sk N .Mo ' R2 

Me 3 Si— N K A 
R 4 




Pentane R.T. 
H 2 1 5 psi 

15 equiv 



N 

III 

^Mo 

Me 3 Sir-N 



Me,Si 



R 





Rj = R 2 = R 3 = H, R 4 = Me (37), 
R, = R 2 = Me, R 3 = R 4 = H (38), 
R, = R 2 - R 3 = H, R 4 = CH 2 CH 3 
and R, = R 3 = Me, R 2 = R 4 = H (39) 



benzene (40) 
toluene (41) 
L-R' = o-xylene (42) 
w-xylene (43) 
p-xylene (44) 
diphenylmethane (45) 
bibenzyl (46) 



Figure 1-14. Molybdenum olefin complexes and the synthesis of arene complexes 

Scope of the Dissertation 
The work embodied in this dissertation reports the latest results toward new 
imido-diamido molybdenum complexes. Chapter 1 is a general introduction to metal 












15 

imides and amides. Chapter 2 discusses pyridine, phosphine, phosphite, and Arduengo 
carbene Lewis base-stabilized imido-diamido complexes and carbon monoxide 
derivatives. Chapter 3 is devoted to the discussion of dihydrogen complexes generated 
from the phosphine complexes discussed in Chapter 2. In Chapter 4, imido-diamido 
butadiene complexes and related reactivity involving the synthesis of alkyne complexes 
are discussed. The reactions of molybdenum olefin and arene complexes with imines, 
ketones, aldehydes, and azobenzene are discussed in Chapter 5. Experimental data are 
recorded in Chapter 6. 

In this work, we have taken advantage of the unique ability of our Ti-loaded 
imido-diamido system to stabilize interesting and rare molybdenum complexes. In a 
rc-loaded system, the excess Ti-donors can act as electron sinks, stabilizing 
high-oxidation-state molecules through p-d overlap if necessary. Oftentimes the 
reactivity observed resembles that of later metals in lower oxidation states. One 
long-term goal of this project is to take advantage of this rc-loaded system to develop 
species that can replace expensive, later metals in chemical syntheses. 












CHAPTER 2 

SYNTHESIS OF LEWIS BASE-STABILIZED MOLYBDENUM COMPLEXES: 

SOURCES OF REACTIVE MOLYBDENUM(IV) 

The dissociation of labile Lewis bases from transition metal centers can generate 
reactive molecules capable of interesting stoichiometric and catalytic reactions that often 
proceed through oxidative-addition and reductive-elimination pathways. 1 We are 
interested in exploring this avenue regarding our molybdenum imido-diamido system and 
have thus prepared a (bis)pyridine complex, phosphine and phosphite complexes, and a 
coordinatively unsaturated Arduengo carbene complex. In this chapter we report the 
synthesis, structure, and initial reactivity studies of these complexes. Some of the 
phosphine complexes reported here react with molecular hydrogen, generating stretched 
dihydrogen complexes. Chapter 3 is devoted to the synthesis and properties of these 
novel dihydrogen complexes. 

Generation of Imido-Diamido Pyridine Complexes 
Synthesis and Characterization of |Mo(NPh)(Py) 2 (o-(Me3SiN)2C 6 H4)] (47) 

Adding excess pyridine to a stirring pentane solution of olefin complex 37 or 38 
resulted in the precipitation of [Mo(NPh)(Py) 2 (o-(Me3SiN) 2 C 6 H4)] (47) as a purple solid 
that was isolated by filtration in high yield (Scheme 2-1). 51 The *H NMR spectrum of 47 
displays a significant broadening of the pyridine protons in the 2 and 6 positions at 20°C. 
At low temperature (-20°C), two distinct doublet resonances are observed in the ! H NMR 
spectrum for these protons: one for the two protons syn to the imido group and the second 
for two protons ami to the imido functionality. The proton resonances corresponding to 



16 



17 

the pyridine ligand protons in the 2 and 6 positions, 3 and 5 positions, and 4 position are 
labeled 2-6, 3-5, and 4, respectively, in the expanded 'H NMR spectrum shown in Figure 
2-2. These observations are consistent with the slow rotation of the pyridine rings about 



the Mo-N bond on the NMR time scale at -20°C. 

Ph 

/ 

N 



Ph 



N 



Me 3 Sk XT II] Ri HI 

H&r**^ - R 2_ Pyridine . Me 3 Si^ >r ^V N 



Me 3 Si^-N' 

/V H ' 




M 



R, = Me, R 2 = H(37), 
R, - R 2 = Me (38) 



Pentane RT MegSip-N N 



47 





Figure 2- 1 . Synthesis of [Mo(NPh)(Py) 2 (o-(Me3SiN)2C 6 H4)] (47) 



V 2 



+ 



A 
H H 



2-6 



J 

"" I — i — r— 1 — i — |— 




2-6 3-5 




)^ m w i »i« . 



-| — i — i — i — r 



"i — I — i — i — i — r 



8.5 



8.0 



7.5 



7.0 



6.5 



6.0 



ppm 



Figure 2-2. Enlarged region of the ] H NMR spectrum of 47 at -20°C 

An X-ray structural analysis was carried out on a single crystal of 47 grown at 20°C 
by layering a saturated toluene solution of 47 with pentane. The crystal data and details 
of the structure refinement are summarized in Table 2-1. A thermal ellipsoid plot of 47 is 
shown in Figure 2-3 with selected bond lengths and angles. The solid-state structure 
reveals a square pyramidal geometry about the molybdenum atom, with the imido ligand 
in the apical position. The Mo-N(4) and N(5) bond lengths of 2.1247(16) A and 



18 



2.1460(16) A, respectively, are consistent with Mo(IV)-pyridine Lewis acid-base 
interactions. The Mo-N(l) bond length of 1.7476(14) A and the C(l)-N(l)-Mo angle of 
166.35(13)° are typical values for molybdenum imido triple-bond interactions."' 27 ' 47 




C20 



Figure 2-3. Thermal ellipsoid plot of 47 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles ("): Mo-N(l) 1.7476(14), Mo-N(2) 2.0779(16), 
Mo-N(3) 2.0637(16), Mo-N(4) 2.1247(16), Mo-N(5) 2.1460(16), 
C(l)-N(l)-Mo 166.35(13), N(3)-Mo-N(4) 88.22(6), N(5)-Mo-N(4) 
80.43(6), N(2)-Mo-N(5) 88.72(6), N(2)-Mo-N(3) 78.30(6). 

The space-filling model of 47, generated from the X-ray study, reveals a sterically 

congested area around the pyridine ligands due to the presence of the SiMe 3 groups 

(Figure 2-4). This steric crowding hinders the rotation of these ligands. 



19 



Table 2-1 . X-ray data 3 for crystal structures 19, 47, and 48 





19 


47 


48 


Chemical formula 


C20H33N3M0S12 


C28H37N5M0S12 


C 2 9H37N 5 MoOSi2 


Formula weight 


467.61 


595.75 


623.76 


Crystal system 


Monoclinic 


Orthorhombic 


Monoclinic 


Space group 


P2,/n 


Pna2, 


C2/c 


u.(Mo-Axi) (mm" 1 ) 


0.652 


0.547 


0.542 


a (A) 


10.3212(4) 


15.5947(8) 


16.2857(8) 


b(A) 


18.0052(7) 


10.3170(5) 


16.9169(8) 


c(A) 


13.3662(5) 


18.4572(9) 


22.086(1) 


pn 


103.774(2) 


- 


94.578(1) 


v c (A 3 ) 


2412.5(2) 


2969.6(3) 


6065.3(5) 


z 


4 


4 


8 


(5) 
w max 


27.50 


27.49 


27.50 


Total reflections 


21116 


19478 


21646 


Uniq. reflections 


5508 


6111 


6965 


R(int) 


0.0300 


0.0272 


0.0930 


Ri [/>2a(/)data] b 


0.0221 


0.0218 


0.0475 


wi? 2 (all data) c 


0.0609 


0.0530 


0.1198 


Larg. diff. peak, hole 


0.380, -0.336 


0.231,-0.222 


0.474, -0.637 



a Obtained with monochromatic Mo Ka radiation (X = 0.71073 A) at 173 K. b /?, = SllFj - 1 
Fj/zlFj. c wi? 2 = {£[w(F 2 - F c 2 ) 2 /I[w(F 2 ) 2 ]} 1/2 . 





















20 




SiMe 3 



Pyridine 



Figure 2-4. Space-filling model of 47. Steric crowding hinders pyridine ligand rotation 
Metal d 2 vs. d° Electronic Configuation and Diamido Ligand Folding 

Although the synthesis of dialkyl complexes 19-23 has been reported, 27 X-ray 
structural studies on these compounds were not carried out at that time. We are interested 
in the solid-state structures of these complexes, and an X-ray study was carried out on a 
single crystal of 19 grown in a pentane solution at -30°C. The thermal ellipsoid plot of 19 
is shown in Figure 2-5. The crystal data and details of the structure refinement are 
summarized in Table 2-1. The Mo-C(19) and Mo-C(20) bond lengths of 2.191 1(19) A 
and 2.2041(19) A, respectively, are within the range expected for metal-carbon single 
bonds. The Mo-N(2) and Mo-N(3) bond lengths of 2.01 17(14) A and 2.0182(14) A, 
respectively, also fall within the expected range. The Mo-N(l) bond length of 1 .7204(14) 









21 

A and the C(l)-N(l)-Mo angle of 171.66(13)° are consistent with a metal-nitrogen 
triple-bond interaction. 




Figure 2-5. Thermal ellipsoid plot of 19 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.7204(14), Mo-N(2) 2.01 17(14), 
Mo-N(3) 2.0182(14), Mo-C(20) 2.2041(19), Mo-C(19) 2.191 1(19), 
C(l)-N(l)-Mo 171.66(13), N(2)-Mo-N(3) 82.73(5), N(2)-Mo-C(19) 
84.86(7), N(3)-Mo-C(20) 85.85(7), C(20)-Mo-C(19) 79.49(8). 

There is a significant difference between the structures of the cf and cf analogues 
involving bonding of the diamide as born out by comparison of the solid-state structures 
of 19 (if) and 47 (cf). In 47 the C 6 H 4 ring, the diamido nitrogens (N(2) and N(3)), and 



22 

the metal center are nearly co-planar. This co-planarity is no longer present in 19, and 
there is an angle of 45.6° between the planes defined by the C 6 H 4 ring and the N(3), Mo, 
and N(2) atoms, as depicted in Figure 2-6. This structural feature is referred to in the 
literature as "ligand folding", and the above-mentioned angle for 19 is called the 
fold-angle. 

Recent density functional theory (DFT) studies attribute ligand folding to 
7i-donation from the NSiMe 3 lone pairs to an appropriate metal-based atomic orbital at 
the d° metal-center for 19 and similar compounds. 52 In these structures, the diamido 
nitrogen atoms remain sp 2 hybridized, and in order to achieve effective p-d overlap, the 
diamido ligand must take on the folded configuration. This bonding situation, which may 
simply be described as a 3 orbital 4e" interaction (Figure 2-7), contributes to the 
n-loading of this molybdenum system. In contrast, there is no ligand folding in 47 (d 2 ) in 
order to avoid a filled-filled interaction between the p and d orbitals (the fold-angle for 47 
is 4.5°). The lack, of ligand folding places the SiMe 3 groups in proximity to the pyridine 
rings, hindering their rotation as mentioned in the previous section. The term 7i-loaded 
applies well to 47 for there are too many ligand n-donors and not enough metal-based 
acceptors. 

We have explored the structure in compounds 19 and 47 theoretically using DFT 
in order to gain more insight into the bonding interactions in these compounds. DFT 
calculations were performed using the Gaussian 98 W program package (see Chapter 6 for 
more details). 53 The model systems consist of the structures shown in Figure 2-8. Steric 
effects of the ligand set were investigated by varying the substitution pattern at the 
diamido nitrogen. The optimized geometries were visualized with gOpenMol, and results 



23 



for cases 19a, 19b. and 19c are shown in Figure 2-9. 54,55 Model system 19c is the most 
accurate for reproducing the experimental structure of 19. The fold-angle increases in the 
order 19a (18.5°), 19b (26.0°), 19c (40.3°) (since there is no C 6 H 4 moiety in the model 
system, the N-C=C-N plane of the diamide was used to calculate the fold-angle). This 
increase in angle can be attributed to the increase in the steric component of the diamido 
ligand in going from model system 19a to 19c. These initial results indicate that attempts 
to directly correlate ligand fold-angle to bond order should be avoided. 



Ph 



Ph 



Me 3 Si N 



Me.Si 



N 




N 




\ 



N 
Me 3 Si 



47 
fold-angle = 4.5° 



fold-angle 



C 6 H 4 plane 




N"";Mo-"/ Me 
Me 3 Si-r-N V 

H 

19 

fold-angle = 45.6° 



N-Mo-N (plane) 



Figure 2-6. Diamido ligand folding in molybdenum imido-diamido complexes 



Me.Si 




Y 



SiMe, 



X/ 

anti-bonding 

non-bonding 
bonding - 



u 



Figure 2-7. Ligand-folding and a general 3 orbital 4e" interaction 



24 

H H H 

/ / / 

H-N H^ H^f 

H H H 

L = Me (19a), L = Py (47a) L = Me (19b) L = Me (19c), L = Py (47c) 

Figure 2-8. Model systems for DFT studies on 19 and 47 

The MOs involved in the 3 orbital 4e~ interaction (Figure 2-7) for 19c have been 
identified. The MO involved in the metal-diamide bonding interaction is the HOMO of 
19c, represented graphically in Figure 2-10. 54 " 55 This MO is best described as interacting 
metal (predominately d x 2 . y 2 and d yz ) and diamido nitrogen p y and/).- atomic orbitals. The 
lowest unoccupied molecular orbital (LUMO) of 19c corresponds to the anti-bonding 
MO for this 3 orbital 4e" interaction as presented in Figure 2-7 (Figure 2-10). The 
nonbonding MO, as depicted in Figure 2-7, has been lowered in energy due to other 
interactions and is the HOMO-3 of 19c shown in Figure 2-10. 

In contrast to the d° system, the HOMO-1 orbital in 47c, which is predominately of 
d x . y character, best represents the lone-pair d electrons on the metal-center. This MO is 
represented graphically in Figure 2-11. The LUMO of 47c is localized on the pyridine 
ligands as shown in Figure 2-11. 

Synthesis and Characterization of [Mo(NPh)/ra/w(Py)2(CO)(0-(Me 3 SiN)2C 6 H4)] (48) 

Exposure of a toluene solution of 47 to dry carbon monoxide gas (ca. 15 psi) 
resulted in the formation of the six-coordinate complex [(Mo(NPh)-/ra/7S-(Py) 2 (CO) 
(o-(Me 3 SiN)2C 6 H4)] (48) (Figure 2-12). An X-ray crystallographic study was carried out 
on single crystals of 48 grown from a concentrated toluene solution. The thermal 



25 








19a, fold-angle = 18.5° 






H& #* 



19b, fold-angle = 26.0° 




1*7yf 



19c, fold-angle = 40.3 C 



Figure 2-9. Optimized geometries for 19a, 19b, and 19c, emphasizing ligand folding 



26 




HOMO of 19c 







LUMOofl9c 




•^ 




HOMO-3ofl9c 



Figure 2-10. Important MO interactions for 19c 



27 





HOMO-1 of 47c 





LUMOof47c 
Figure 2-11. Important MO interaction for 47c 

ellipsoid plot of 48 is shown in Figure 2-13 along with selected bond lengths and angles. 
The crystal data and details of the structure refinement are summarized in Table 2-1. 

Complex 48 adopts a distorted octahedral geometry. The Mo-N(l) bond length of 
1.778(3) A and the C(l)-N(l)-Mo angle of 162.5(3)° are consistent with a 
molybdenum-nitrogen triple-bond interaction. The Mo-N(4) and N(5) bond lengths of 
2.207(3) A and 2.217(3) A, respectively, are within the expected range for a 
six-coordinate molybdenum pyridine adduct. An important feature in 48 is the trans 
geometry adopted by the two pyridine ligands as well as the trans arrangement of one 
amide and the carbonyl ligand, presumably a result of combined steric and electronic 



28 



effects. For example, this trans arrangement of amido and CO ligand allows for a 3 
orbital 4e" interaction between the filled amido p orbital, the appropriate metal d orbital, 
and the CO n orbital. Furthermore, the trans orientation of the pyridine ligands keeps 
them away from each other and away from the steric bulk of the SiMe 3 groups. 

Ph 



Ph 



Me.Si 



N 




N Mo^x, 

N 



tf 



\ 
Me.Si 



Me.Si 




Toluene 



*A* 



CO(ca. 15 psi) 7 p y 




Me.Si 



47 



48 



Figure 2-12. Synthesis of [(Mo(NPh)/ram(Py) 2 (CO)(o-(Me 3 SiN)2C 6 H4)] (48), Py = 
pyridine 

Two resonances for the inequivalent SiM? 3 groups are observed at 0.32 ppm and 

0.37 ppm in the 'H NMR spectrum of 48, consistent with the solid-state structure. The 

equivalency of the pyridine protons in the 2 and 6 positions in the 'H NMR spectrum at 

20°C indicate that rotation about the pyridine molybdenum bond is not hindered at this 

temperature on the NMR time scale in this six-coordinate complex. This is interesting 

when compared to the hindered rotation of the pyridine rings in five-coordinate 47 and is 

an unusual example of a coordinatively saturated system (48) being less sterically 

hindered than an unsaturated system (47). A stretching frequency of 1913 cm" 1 in the IR 

spectrum of 48 has been assigned to the CO ligand (free carbon monoxide has a C-0 

stretch at 2143 cm" 1 ), indicating a considerable amount of back bonding from the metal to 

the carbon monoxide ligand. 



29 




C27 



Figure 2-13. Thermal ellipsoid plot of 48 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.778(3), Mo-N(2) 2.128(3), 
Mo-N(3) 2.136(3), Mo-N(4) 2.207(3), Mo-N(5) 2.217(3), Mo-C(29) 
1.993(5), C(29)-0(29) 1.158(5), 0(29)-C(29)-Mo 175.2(4), N(4)-Mo-N(5) 
169.52(12), N(2)-Mo-N(3) 78.10(13), C(l)-N(l)-Mo 162.5(3). 












30 

C-N Activation of an Amido Ligand: Synthesis of Imido-Bridged 49 from 47 

Monomeric 47 is stable at 20°C in the solid-state for extended periods. However, 
when heated to 80°C in toluene, 47 converted cleanly to bimetallic 49 with loss of 
pyridine in 2 h (Figure 2-14). 51 This air sensitive, diamagnetic complex is thermally 
stable in solution for extended periods, even at the temperature required for synthesis. A 
single crystal of 49 was grown from a pentane/dichloromethane solution at -30°C. 
Compound 49 crystallizes with two molecules of dichloromethane. An X-ray diffraction 
study shows that 49 contains two molybdenum atoms bridged by two phenyl imido 
groups as well as a Me 3 SiN-C 6 H 4 ligand (Figure 2-15). This unusual Me 3 SiN-C 6 H 4 
group is apparently formed by cleavage of one NSiMe 3 group from an 0-(Me 3 SiN) 2 C 6 H4 
ligand. The NSiMe 3 group that was cleaved remains as an additional terminal imido 
ligand on one of the molybdenum atoms. The formal oxidation state at each metal center 
is best described as Mo(V). The Mo(l)-Mo(2) distance of 2.5669(4) A, although short 
for a Mo-Mo single bond, indicates the existence of a metal-metal bond and accounts for 
the observed diamagnetism of 49. 56,57 Four upfield resonances, assigned to the four 
inequivalent Me 3 Si groups, are observed in the *H NMR spectrum of 49, consistent with 
the structure as determined by X-ray crystallography. The crystal data and details of the 
structure refinement for 49 are summarized in Table 2-2. 

The unusual aromatic C-N bond cleavage reaction that is observed during the 
pyrolysis of 47 is presumably driven by the formation of the Mo-N triple bond and 
demonstrates the reactivity of the Mo(IV) moiety towards oxidation. Reactions providing 
a straightforward example of C-N single-bond activation, a most desirable 
transformation, are rare. 58 The metal-mediated rupture of C-N bonds is, for the most part, 
limited to strained amines' 9 or amidines. 60 Activation of nonactivated substrates such as 



31 

aniline and the ring opening of pyridine have been observed with highly reactive, 
trivalent, Group 5 metal complexes. A slight variation on this theme involves the 
recently reported C-N bond cleavage reactivity of a Nb(II) cluster upon ligand 
replacement by anionic amides. 63 An observation related to this reaction type, made in 
1985 by Chisholm, 64 involved the isolation of a carbide/imide cluster that may have 
arisen via degradation of an amido ligand. 

Ph Ph Ph 

Me 3 Si N • Me 3 Sk W SiMe 3 

m.. Jl 1 /=^ Toluene /r\ <fc /A\ / N V^ 





Me 3 Si K^J Me 3 Si / 

47 
Figure 2-14. C-N activation of the o-(Me 3 SiN)2C 6 H4 ligand in 47 

Generation of Imido-Diamido P(OMe) 3 Adducts 
Synthesis and Characterization of [Mo(NPh)(P(OMe)3)3(o-(Me3SiN)2C 6 H4)] (50) 

From a pentane solution of olefin complex 37 or 38 treated with 4.0 equiv of 
P(OMe) 3 precipitated a red-brown powder of [Mo(NPh)(P(OMe) 3 )3(o-(Me 3 SiN) 2 C 6 H4)] 
(50), which was isolated in high yield (Figure 2-16). A single crystal X-ray study was 
carried out on a suitable crystal of 50 grown from a toluene/pentane solution at low 
temperature. The crystal data and details of the structure refinement are summarized in 
Table 2-2. In the thermal ellipsoid plot of 50, the geometry about molybdenum is best 
described as a distorted octahedron, and the olefin from the starting material has been 
displaced by three trimethyl phosphite ligands (Figure 2-17). The phosphite ligands take 
up a meridional bonding motif, and the Mo-P(l), Mo-P(2), and Mo-P(3) bond lengths of 
2.5090(8) A, 2.4509(8) A, and 2.4972 A, respectively, are as expected for a complex of 



32 



this nature. The molybdenum imido interaction is consistent with a metal-nitrogen triple 
bond. 

C32 

C22_ C23 




C37 



Figure 2-15. Thermal ellipsoid plot of 49 (50% probability thermal ellipsoids). The 
solvating dichloromethane molecules have been omitted for clarity. 
Selected bond lengths (A) and angles (°): Mo(l)-Mo(2) 2.5669(4), 
Mo(l)-C(34) 2.179(3), Mo(2)-N(7) 2.047(2), Mo(2)-N(6) 1.745(2), 
Mo(2)-N(5) 2.285(3), Mo(l)-N(l) 1.955(2), Mo(l)-N(2) 1.866(2), 
Mo(2)-N(l) 2.007(2), Mo(2)-N(2) 2.061(2), Mo(l)-N(l)-Mo(2) 80.74(9), 
Mo(l)-N(2)-Mo(2) 81.48(9), Mo(2)-N(6)-Si(3) 170.50(17). 



Ph 



N 



Me.Si- 



R, 



Ph 

/ 

Me 3 Si^ N 

N JJL^P(OMe) 3 Ri R 2 




./ o. N ^ Mo O-R, 4.0P(OMe) 3 i^Md 

Me 3 Si— N J/* 2 i J ±+ (MeO) 3 P t T^ \\ 

fH PentaneRT /Z\— N p ( 0Me )3 

H / // \ H H 

\/ Me 3 Si 



R, = Me,R 2 = H(37), 
R, = R 2 = Me (38) 



50 



Figure 2-16. Synthesis of [Mo(NPh)(P(OMe) 3 ) 3 (o-(Me 3 SiN) 2 C 6 H 4 )] (50) 



33 

Although the SiMe3 groups of 50 are inequivalent in the solid-state, only one SiMe^ 
resonance is observed for these protons in the ! H NMR spectrum from 20°C (0.62 ppm, 
CyDg) to -70°C. A high degree of fluctionality in solution, most likely involving 
phosphite dissociation and exchange, could be responsible for the formation of a 
five-coordinate complex where the SiMes groups are equivalent, accounting for this 
observation. In the (bis)pyridine complex (47), steric interactions hinder the rotation of 
the pyridine ligands about the Mo-N bonds. The steric interactions in 50 should help 
encourage phosphite exchange and, in this way, generate a coordinatively unsaturated 
metal center. Steric interactions such as this one will become a recurring theme 
throughout this chapter. 

The phosphite complex is rather unstable and begins to decompose in solution at 
20°C within hours. Compound 50 even decomposes in the solid-state at -30°C over 
months. The decomposition product was isolated as an air and thermally sensitive, black, 
toluene-insoluble powder. The thermal sensitivity and the insolubility of this compound 
have made it very difficult to characterize. We have hypothesized that this compound 
may be an oligomeric species with the formula [Mo(NPh)(o-(Me3SiN)2C6H 4 )] x , but have 
no solid evidence with which to substantiate this claim other than a 'H NMR spectrum. 46 
This unknown material will be henceforth referred to as X. 
Synthesis and Characterization of [Mo(NPh)(P(OMe)3)2CO(o-(Me 3 SiN)2C 6 H4)] (51) 

When freshly generated 50 was dissolved in a minimum amount of toluene and 
treated with CO gas (ca. 15 psi), [Mo(NPh)(P(OMe) 3 )2CO(o-(Me3SiN) 2 C 6 H4)] (51) 
formed and was isolated in excellent yield. Single crystals of 51 were grown from a 
toluene solution, and an X-ray study was carried out on a suitable crystal. The crystal 
data and details of the structure refinement are summarized in Table 2-2. The thermal 



34 

ellipsoid plot of 51 is shown in Figure 2-18. The overall geometry around the metal is 
distorted octahedral, and the structure type is similar to that of 48. In this instance, the 
phosphite ligands are mutually trans, presumably for the same reasons discussed for the 
pyridine ligands in 48. 




C24 



C23 



Figure 2-17. Thermal ellipsoid plot of 50 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.771(2), Mo-N(2) 2.161(2), 
Mo-N(3) 2.190(2), Mo-P(l) 2.5090(8), Mo-P(2) 2.4509(8), Mo-P(3) 
2.4972(8), N(l)-Mo-N(2) 174.40, C(l)-N(l)-Mo 163.2(3), N(3)-Mo-P(2) 
165.99(6), P(3)-Mo-P(l) 168.10(3). 

The 'H and 13 C NMR spectra of 51 are highly fluxional at 20°C (C 7 D 8 ). In the 'H 

NMR spectrum at 20°C, a single resonance is observed for the SiMe 3 groups at 0.66 ppm 

along with a doublet resonance at 3.09 ppm (V P . H = 6.5 Hz) for the P(OMe) 3 ligands. At 



35 

low temperature (-50°C, CyDg), two resonances are observed for the SiMe^ groups at 0.66 
ppm and 0.75 ppm in the *H NMR spectrum. Phosphite dissociation from 51 at 20°C 
could produce a five-coordinate species with equivalent SiMe3 groups, accounting for 
this behavior. 

The coalescence of the SiMe3 peaks in the ! H NMR spectrum takes place at 5°C. 
The activation energy for this process can be calculated using the simple two-site 
exchange equation shown in Equation 2-1, where N is Avogadro's number, T c is the 
temperature of coalescence, and Sv is the maximum chemical shift difference between 
the two resonances in question. 65 Using Equation 2-1 a value of 58.73 KJ/mol (14.04 
Kcal/mol) is calculated for this process, where Sv = 24 Hz. 

AG % I RT C =ln ( V2 (R / TiNh)) + In (T c / Sv ) Equation 2- 1 
A peak at 1959 cm" 1 corresponding to the carbonyl ligand stretch was observed in 
the IR spectrum of 51. The carbonyl stretching frequency for 51 is 46 cm" 1 higher than 
that for 48, implying a stronger CO bond in 51. This is not surprising as the phosphite 
ligands are weaker Lewis bases than pyridine, and 51 will therefore have less electron 
density at the metal center available for back bonding to the carbonyl ligand. 

Generation of Imido-Diamido PMe3 Adducts 
Synthesis and Characterization of [Mo(NPh)(PMe 3 )3(o-(Me3SiN)2C 6 H4)] (52) 

The [Mo(NPh)(PMe3)3(o-(Me 3 SiN)2C6H4)] (52) complex was synthesized by Dr. 
Carlos Ortiz, a previous graduate student, through a route identical to that used in the 
preparation of 50. 66 The results of the X-ray crystal structure study on this complex are 
included here, as they are relevant to structural discussions in this chapter. The thermal 
ellipsoid plot of 52 is shown in Figure 2-19 along with selected bond lengths and angles. 






36 

The overall geometry is distorted octahedral and similar to that of 50. The bond lengths 
and angles associated with the phenyl imido group are as expected for a metal-imido 
triple-bond interaction (Mo-N(l) 1.7851(14) A, C(l)-N(l)-Mo 169.77(13)°). The rest of 
the bond lengths are unexceptional in that they all fall within the expected range. There 
is, however, a significant difference of approximately 0.1 A between the Mo-N(2) and 
Mo-N(3) bond lengths, 2.2325(14) A and 2.1408(14) A respectively, in this complex. 
Elongation of Mo-N(2) has been attributed to a filled-filled interaction between a 
nitrogen (N(2) in 52) p orbital and a metal d orbital of appropriate symmetry. This 
filled-filled interaction is presumably alleviated in 48 due to the n-accepting carbonyl 
ligand, and the corresponding molybdenum nitrogen amide bond lengths in 48 differ by 
approximately 0.01 A. However, such arguments should be made with caution as steric 
effects may also have an impact on bond length. Consider, for example, complex 53 an 
analogue to 52 where a SiMe3 group has been replaced with a proton (Figure 2-20). In 53 

the Mo-N(3) bond length (2.0873(16)A) is 0.05 A shorter than the corresponding length 

• 

in 52 (2.1408(14)A), presumably due to loss of steric clout upon replacing SiMe 3 with H. 

The origin of complex 53 will be discussed in-depth in Chapter 3. The X-ray crystal 

study of 53 was first reported by Dr. Carlos Ortiz. It is included here for the purposes of 

this discussion. 66 

Synthesis and Characterization of [Mo(NPh)(PMe 3 ) 2 CO(0-(M e 3 SiN) 2 C 6 H 4 )] (54) 

Exposure of a toluene solution of 52 to dry carbon monoxide gas (ca. 15 psi) 
resulted in the formation of the six-coordinate complex [(Mo(NPh)(PMe 3 ) 2 (CO) 
(o-(Me 3 SiN) 2 C 6 H4)] (54) (Figure 2-21). The structure of 54 has been assigned by 'H, 



37 



C3 SS 



C22 




C24 



Figure 2-18. Thermal ellipsoid plot of 51 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.7882(13), Mo-N(2) 2.1566(12), 
Mo-N(3) 2.1242(13), Mo-C(19) 2.01 12(16), Mo-P(l) 2.5291(4), Mo-P(2) 
2.4784(4), 0(1)-C(19) 1.145(2), N(l)-Mo-N(3) 166.33(5), N(2)-Mo-C(19) 
162.00(6), P(l)-Mo-P(2) 170.451(15), C(l)-N(l)-Mo 161.78(12). 



38 



Table 2-2. X-ray data 3 for crystal structures 49, 50, and 51 



49 


50 


51 


Chemical formula C^HsgNTMoaSi^CHaC^ C 


27H54N 3 Mo09P3Si2 


C25H45N3Mo0 7 P2Si2 


Formula weight 1 1 24 . 04 


809.76 


713.70 


Crystal system Monoclinic 


Triclinic 


Monoclinic 


Space group P2]/n 


PI 


P2,/c 


\i(Mo-Ka) (mm" 1 ) 0.805 


0.578 


0.581 


a (A) 14.4655(8) 


10.1800(5) 


17.2665(6) 


b(A) 23.572(1) 


10.4700(5) 


11.3438(4) 


c(A) 15.9891(9) 


18.6036(9) 


18.3306(7) 


a(°) 


91.681(2) 


- 


P(°) 104.390(1) 


92.692(2) 


104.640(2) 


y(°) 


104.317(2) 


- 


V c (A 3 ) 5280.8(5) 


1917.4(2) 


3473.8(2) 


Z 4 


2 


4 


©max° 27.50 


27.50 


27.50 


Total reflections 37951 


17160 


30213 


Uniq. reflections 12085 


8561 


7876 


^(int) 0.0512 


0.0298 


0.0328 


ft, [7>2o(/)data] b 0.0384 


0.0416 


0.0222 


w# 2 (all data) c 0.0930 


0.1021 


0.0641 


Larg. diff. peak, hole 0.807, -0.694 


1.063,-0.864 


0.326, -0.360 


a Obtained with monochromatic Mo Ka radiation 

—. II , 1 „ 1 /» _ -) "(-> 1 -. 1 /-* 


(X = 0.71073 A) at 173 K. b fl, = sIfJ - 1 



F c I/eIfJ. c wi? 2 = {£[w(F 2 - F c 2 ) 2 /E[w(F 2 ) 2 ]} 1/2 . 



39 



C24 



C4 




C26 

Figure 2-19. Thermal ellipsoid plot of 52 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.7851(14), Mo-N(2) 2.2325(14), 
Mo-N(3) 2.1408(14), Mo-P(l) 2.5280(5), Mo-P(2) 2.5461(5), Mo-P(3) 
2.5534(5), C(l)-N(l)-Mo 169.77(13), N(l)-Mo-N(3) 176.91(6), 
P(3)-Mo-P(2) 169.383(16), N(2)-Mo-P(l) 171.381(4). 












C24 



C22 



C19 



lC23 



P3 



Si1 



C21 



<7 



N2 



C7 



40 


C4 






C5 s% 




^nC3 




C6 ^jfc 


pTci 


^C2 




N1 d 
Mo 1 


C13 

' ^ C16 

P1 L^r^y 


18* 






«tVp2 




"C20 1 
N3l 




1 C15 N 





C18 



C17 



C12 



C8 



C11 



C9 



Figure 2-20. Thermal ellipsoid plot of 53 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.7972(1), Mo-N(2) 2.2176(15), 
Mo-N(3) 2.0873(16), Mo-P(l) 2.5385(6), Mo-P(2) 2.4612(6), Mo-P(3) 
2.5204(5). 















41 

C, and P NMR spectroscopy and is consistent with that shown in Figure 2-21 . At 
20°C two resonances are observed for the inequivalent SiMe 3 groups at 
0.51 ppm and 0.60 ppm in the 'H NMR spectrum. One broad resonance at -15.2 ppm is 
observed for the equivalent phosphines in the 31 P{'H} spectrum of 54 at 20°C and the 
carbonyl carbon resonates at 260.2 ppm in the 13 C NMR spectrum at that temperature. 
The carbonyl stretch in the IR spectrum of 54 is observed at 1925 cm" 1 . The electron 
density at the metal center, determined primarily by the PMe 3 ligands, is thus somewhere 
between that in 47 and 51. 

/ Ph 7 Ph 

Me 3 Si ^ mi *w Me 3 Si v N 

N ,„ lll..oPMe 3 v, IIUPMe, 

/ Mo Toluene r Mo 

(Me 3 P,^PMe 3 CO (ca. 1 5 psi) ^vle/^CO 

"""" \ 
Me 3 Si 

54 

Figure 2-2 1 . Synthesis of [(Mo(NPh)(PMe 3 ) 2 (CO)(o-(Me 3 SiN) 2 C 6 H 4 )] (54) 

Generation of Imido-Diamido PMe 2 Ph Adducts 
Synthesis and Characterization of [Mo(NPh)(PMe 2 Ph) 2 (0-(Me 3 SiN) 2 C 6 H4)] (55) 

The olefin fragment in complexes 37 and 38 was displaced with 2.0 equiv of 
PMe 2 Ph. The product in this instance was the bis(phosphine) complex 
[Mo(NPh)(PMe 2 Ph) 2 (o-(Me 3 SiN) 2 C6H4)] (55) that precipitated from solution as a green 
solid, and was isolated by filtration (Figure 2-22). Single crystals of 55 were grown from 
a concentrated toluene solution at low temperature. An X-ray study was carried out on a 
suitable crystal, and the thermal ellipsoid plot of 55 is shown in Figure 2-23. The 
relevant details of the structure refinement are summarized in Table 2-3. 








42 

The geometry about the molybdenum in 55 is best described as distorted square 
pyramidal. The atoms N(2), N(3), P(2) and N(l) make up the base of the square pyramid, 
as defined by the two biggest metal-centered angles in the molecule, N(l)-Mo(l)-N(3) 
1 5 1 .03(8)° and N(2)-Mo( 1 )-P(2) 1 64.67(5)°. The remaining P( 1 ) occupies the apical 
position. 

The formulation of the molecule as a bis(phosphine) complex is also supported by 
integration of the SiMe 3 and ?Me 2 ?h resonances in the 'H NMR spectrum of 55. The 
structure of complex 55 is fluxional in solution, and one resonance is observed for both 
SiMe 3 groups at 0.45 ppm along with one resonance for the two ?Me 2 Ph ligands at 0.94 
ppm (br) in the *H NMR spectrum (-65°C, C 7 H 8 ). 

When 55 was generated by treatment of an olefin complex with 2.0 equiv PMe 2 Ph, 
X was simultaneously formed in small amounts. Using 4.0 equiv PMe 2 Ph helped 
discourage, but did not completely eliminate the formation of X. Complex 55 was, 
however, stable when stored for months at -30°C under an inert atmosphere. 

Ph Ph 

/ / 

Me 3 Sk x . HI Ri Me 3 S \ IH/PMe 2 Ph R, R 2 

hf A PentaneRT JCJ\ PMe 2 Ph + { 

H SiMe 3 H H 

R, = Me, R 2 = H (37), 

Rj = R 2 = Me (38) 55 

Figure 2-22. Synthesis of [Mo(NPh)(PMe 2 Ph) 2 (o-(Me 3 SiN) 2 C 6 H 4 )] (55) 




43 




Figure 2-23. Thermal ellipsoid plot of 55 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo(l)-N(l) 1.768(2), Mo(l)-P(l) 
2.3968(6), Mo(l)-P(2) 2.51 13(6), Mo(l)-N(2) 2.174(2), Mo(l)-N(3) 
2.073(2), N(l)-Mo(l)-N(3) 151.03(8), N(l)-Mo(l)-N(2) 107.33(8), 
N(3)-Mo(l)-P(l) 1 14.38(5), N(2)-Mo(l)-P(2) 164.67(5), C(l)-N(l)-Mo(l) 
168.89(17), N(3)-Mo(l)-N(2) 77.15(7), N(l)-Mo(l)-P(l) 94.55(6), 
N(2)-Mo(l)-P(l) 86.60(5), N(l)-Mo(l)-P(2) 87.73(6), N(3)-Mo(l)-P(2) 
88.30(5), P(l)-Mo(l)-P(2) 95.20(2). Complex 55 crystallized with two 
molecules in the asymmetric unit and only one is shown here. 

Synthesis and Characterization of [Mo(NPh)(PMe 2 Ph)2CO(o-(Me3SiN)2C 6 H 4 )] (56) 

When the coordinatively unsaturated 55 reacted with CO gas (ca. 15 psi), the 
carbonyl adduct [Mo(NPh)(PMe 2 Ph)2CO(o-(Me3SiN) 2 C 6 H4)] (56) formed and was 
isolated as a red/purple solid (Figure 2-24). Two resonances are observed for the 



44 

inequivalent SiM? 3 groups in the 'H NMR spectrum of 56 at 0.19 ppm and 0.55 ppm 
(C 6 D 6 , 20°C). Only one resonance is observed for the four ?Me 2 Ph ligand methyl groups 
at 1 .20 ppm in the 'H NMR spectrum (C 6 D 6 , 20°C). A very broad resonance is observed 
in the P NMR spectrum at -7.0 ppm for the PMe 2 Ph phosphorus group. Based on these 
observations, a phosphine exchange pathway at 20°C can account for the equivalent 
PMe 2 Ph methyl groups. A carbonyl stretching frequency of 1921 cm" 1 is observed in the 
IR spectrum of 56. This frequency is slightly lower than that of complex 54 (1925 cm" 1 ), 
and is surprising in that PMe 2 Ph is slightly more electron withdrawing than PMe 3 . We 
would therefore expect the stretching frequency for 56 to be higher than that for 54. 

Ph Ph 



/ 



/ 





n e- N Me 3 Si x N 

Me 3 S \ NI/PMe 2 Ph Toluene N ^-^ p Me 2 Ph 

V %Me 2 Ph COtoHprf) "*r±\* 
SiMe 3 

55 56 

Figure 2-24. Synthesis of [Mo(NPh)(PMe 2 Ph) 2 CO(o-(Me 3 SiN) 2 C 6 H 4 )] (56) 

Synthesis and Characterization of [Mo(NPh)(DMPE)(0-(Me 3 SiN) 2 C 6 H 4 )-u-(DMPE) 

Mo(NPh)(DMPE)(o-(Me 3 SiN) 2 C 6 H 4 )](57) 

When a pentane solution of olefin complex 37 or 38 was treated with 3.0 equiv of 
DMPE (l,2-bis(dimethylphosphino)ethane), dimeric [Mo(NPh)(DMPE) 
(o-(Me 3 SiN) 2 C 6 H4)-u-(DMPE)Mo(NPh)(DMPE)(o-(Me 3 SiN) 2 C 6 H4)] (57) precipitated 
from solution as a red powder and was isolated by filtration (Figure 2-25). The dimeric 
nature of 57 was confirmed by a single crystal X-ray study. The thermal ellipsoid plot 57 
is shown in Figure 2-26 with selected bond lengths and angles. The details of the 
structure refinement are summarized in Table 2-3. The metal-centers Mo and MoA are 



45 



related by an inversion center. The geometry at Mo in 57 is best described as distorted 
octahedral and is very similar to that in 50 and 52. The phosphorus atoms are 
coordinated to the Mo center in a meridional array, also similar to what is observed for 
complexes 50 and 52. The smallest metal-centered angles are the P(2)-Mo-P(l) and 
N(2)-Mo-N(3) angles of 78.170(15)° and 77.67(5)°, respectively. The acuteness of these 
angles can be attributed to the chelating nature of the ligands. All other bond lengths and 
angles are unexceptional. 



Me 3 Si 




R! = Me,R 2 = H(37), 
R, = R 2 = Me (38) 



3.0 DMPE 
Pentane R.T. 



Me.Si 




SiMe, 



Ph 



A ^~ ?/ "Z "S 

Me 3 Sr N ^ P^ 



57 




SiMe, 



R{ H 

Figure 2-25. Synthesis of [Mo(NPh)(DMPE)(o-(Me 3 SiN) 2 C 6 H4)-u-(DMPE) 
Mo(NPh)(DMPE)(o-(Me 3 SiN) 2 C 6 H 4 )](57) 

Complex 57 reacts with carbon monoxide gas giving a mixture of products by 'H 
NMR spectroscopy. A CO stretch appears at 1927 cm"' in the IR spectrum of the crude 
reaction product. This confirms the presence of a metal carbonyl, however the exact 
nature of the coordination environment about the metal-center is not known at this time. 



46 




Figure 2-26. Thermal ellipsoid plot of 57 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.7914(12), Mo-P(l) 2.4890(4), 
Mo-P(2), 2.4887(4), Mo-P(3) 2.5710(4), Mo-N(2) 2.2061(12), Mo-N(3) 
2.1422(12), C(l)-N(l)-Mo 171.89(12), N(l)-Mo-N(3) 174.94(5), 
P(2)-Mo-P(l) 78.170(15), N(2)-Mo-N(3) 77.67(5), P(2)-Mo-N(2) 
162.68(3), P(l)-Mo-P(3) 175.229(14). All atoms labeled A are symmetry 
related to non A labeled atoms by a center of inversion. 

Generation of an Imido-Diamido Arduengo Carbene (Imidazol-2-Ylidene) Adduct 

Stable and isolable imidazol-2-ylidenes were first discovered by Arduengo in 
1 99 1 . These carbenes display an uncanny ability to coordinate to transition metals 
through a-donation from the carbene to the metal, and back bonding to the carbene is 
negligible. Transition metals modified with imidazol-2-ylidene ligands have recently 
found widespread use in catalytic transformations such as the Heck coupling reaction, 
hydrogenation and hydro formylation of olefins, hydrosilation, olefin metathesis, 
copolymerization of ethylene and CO, and polymerization of alkynes. Some recent 



47 

reviews have thoroughly treated this subject matter. 68,69-70 Our molybdenum 
imido-diamido fragment can stabilize Schrock-type carbene complex 33. We were 
interested in exploring the structure of an imidazol-2-ylidene derivative of our 
imido-diamido system. To this day only carbene complexes of low-valent molybdenum 
in octahedral geometries have been described in the literature. 68,71 We have been able to 
synthesize a complex of this type, and the synthesis and initial structural studies will be 
discussed here. This is both the first instance of a molybdenum imidazol-2-ylidene 
complex with a tetrahedral-like structure and the first instance of a molybdenum (IV) 
imidazol-2-ylidene complex. 

When a benzene solution of 55 was treated with 1 .0 equiv of imidazol-2-ylidene 58 
(IMes) [Mo(NPh)IMes(o-(Me 3 SiN) 2 C 6 H4)] (59) formed in high yield as determined by 
H NMR spectroscopy (Figure 2-27). Although complex 59 was also generated by 
treatment of 52 with 58 the crude reaction product was not as pure as when 55 was used 
as the starting material. Single crystals of 59 grew from C 6 D 6 NMR samples generated 
by treatment of 52 and 55 with 58, and a single crystal X-ray study was carried out on an 
appropriate crystal. The thermal ellipsoid plot of 59 and selected bond lengths and angles 
are presented in Figure 2-28. The details of the structure refinement are summarized in 
Table 2-3. 

One interesting feature of 59 is the lack of phosphine coordination. The IMes 
ligand is a strong a-donor and bulky enough to displace and inhibit phosphine 
coordination. The geometry at the metal-center is best described as distorted tetrahedral. 
The largest deviation from the ideal tetrahedral angle of 109.5° involves the metal 
centered angle N(2)-Mo(l)-N(3) of 78.77(10)°. The chelating nature of the diamido 



48 

ligand is responsible for the acuteness of this angle. The C(l)-N(l)-Mo(l) angle of 
161.6(2)° and the N(l)-Mo(l) bond length of 1.740(3) A agree well with a metal-nitrogen 
triple-bond interaction. The metal carbene (Mo(l)-C(19)) bond length of 2.178(3) A in 
59 is approximately 0.1 0A shorter than in octahedral molybdenum (0) analogues and is 
similar to that of Fisher carbene (CO) 5 Mo=C(OR)SiPh 3 (2. 1 5(2) A). 72 While the metal to 
carbene bond lengths of 59 and (CO) 5 Mo=C(OR)SiPh 3 are similar, we believe that the 
short molybdenum to carbene length in 59 is a result of the high oxidation state and the 
distorted tetrahedral geometry and not a result of substantial back bonding. 

Complex 59 is paramagnetic as revealed by "H NMR spectroscopy. All protons in 
59 are represented by broad singlets via 'H NMR spectroscopy, and the number of peaks 
and relative integration are consistent with the structure of 59 reported here. The 
tentative assignment of the *H NMR spectrum is included in the experimental section. 

We attribute the paramagnetic nature of 59 to a very small energy difference 
between the two lowest energy d orbitals, allowing for thermal population of both orbitals 
at 20°C giving a paramagnetic complex. Magnetic susceptibility measurements will shed 
some light on this situation. 



Me.Si 



Ph 

N 
\ JII/PMe 2 Ph 
N-, Mo N + 

PMe.Ph 





Ph \\ 

\ 

N 
N^ Benzene Me 3Si, ||| 





- PMe 7 Ph 




Figure 2-27. Synthesis of [Mo(NPh)IMes(o-(Me 3 SiN) 2 C 6 H 4 )] (59) 



49 



C38 




C29 



Figure 2-28. Thermal ellipsoid plot of 59 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo(l)-C(19) 2.178(3), Mo(l)-N(l) 
1.740(3), Mo(l)-N(2) 2.074(2), Mo(l)-N(3) 2.031(3), C(19)-N(4) 1.371(4), 
C(19)-N(5) 1.369(4), C(20)-C(21) 1.334(4), N(l)-Mo(l)-N(3) 119.45(11), 
N(l)-Mo(l)-N(2) 1 16.41(1 l),N(2)-Mo(l)-N(3) 78.77(10), 
N(l)-Mo(l)-C(19) 107.3 1(1 l),N(3)-Mo(l)-C(19) 123.51(11), 
N(2)-Mo(l)-C(19) 108.02(1 1). Complex 59 crystallized with two 
molecules in the asymmetric unit and only one is shown here. 



50 



Table 2-3. X-ray data 3 for crystal structures 55, 57, and 59 





55 


57 


59 


Chemical formula 


C 3 4H49N3MoP 2 Si2 


C54H,02N 6 MO2P6Si4 


C 3 9H48N 5 MoSi2 


Formula weight 


713.82 


1325.48 


738.94 


Crystal system 


Monoclinic 


Monoclinic 


Orthorhombic 


Space group 


P2,/n 


P2,/n 


Pbca 


u(Mo-£a) (mm" 1 ) 


0.539 


0.626 


0.433 


a (A) 


26.355(2) 


10.9359(4) 


18.3689(8) 


*(A) 


10.3180(7) 


16.6758(7) 


21.069(2) 


c(A) 


27.297(2) 


18.8773(8) 


40.047(2) 


pn 


100.475(2) 


102.930(2) 


- 


v c (A 3 ) 


7299.4(9) 


3355.3(2) 


15499(2) 


z 


8 


2 


16 


'-'max 


27.50 


27.50 


27.03 


Total reflections 


63955 


29611 


110986 


Uniq. reflections 


16703 


7668 


15985 


fl(int) 


0.0447 


0.0317 


0.0812 


/?, [/>2a(/)data] b 


0.0312 


0.0210 


0.0425 


wR 2 (all data) 


0.0756 


0.0542 


0.1016 


Larg. diff. peak, hole 


0.387, -0.407 


0.323,-0.412 


0.963, -0.935 



a Obtained with monochromatic Mo Ka radiation (X = 0.71073 A) at 173 K. b R\ = ZlFj 

Fj/zlFj. c w/? 2 = {IKF 2 - F c 2 ) 2 /S[w(F 2 ) 2 ]} 1/2 . 












Summary 



The isolation of molybdenum imido-diamido adducts of pyridine, PMe 3 , PMe 2 Ph, 
P(OMe 3 ) 3 , DMPE, and IMes have been discussed. Through the study of these 
compounds a general theme involving steric crowding at the metal center was mentioned 
as was that concerning 7i-loading. 



51 

The DFT study on complex 19 revealed the nature of the diamido ligand folding in 
this d° system. In contrast, DFT studies showed that the absence of ligand folding in 
complex 47 was due to a lack of empty d metal orbitals available for bonding. An 
interesting effect of this lack of folding is the hindered rotation of the pyridine rings. 
Complex 47 is also unique when compared to the tris complexes 50, 52, 53, and 57. 
From a steric stand-point, we have no concrete explanation as to why bis and not tris 
coordination of pyridine is preferred. 

The structures of complexes 50, 52, 53, and 57 are very similar. All 
phosphorus-based ligands bind to molybdenum in a meridional fashion, and the 
coordination sphere of each is best described as distorted octahedral. Complex 57 stands 
out in this group, for the two metal centers in the dimer are linked by a DMPE moiety. 

Complexes 55 and 59 highlight the steric interactions in this system. The PMe 2 Ph 
ligand used in the preparation of 55 has a larger cone angle 73 (cone angle (6°) PMe 2 Ph - 
122) than the phosphine or phosphites used in the preparations of 50, 52, 53, and 57 (0° 
PMe 3 =118,0° P(OMe) 3 = 107, 0° DMPE - 107). This larger cone angle accounts for 
the bis coordination of PMe 2 Ph in 55. Sterically, the IMes ligand in complex 59 is often 
compared with P(Cy) 3 (Cy = cyclohexyl, 0° P(Cy) 3 = 170). The large steric clout of IMes 
is responsible for the displacement of all phosphines from the starting material in the 
preparation of 59. 

Initial reactivity of complexes 47, 50, 52, and 55 with carbon monoxide gas was 
outlined, and a summary of these complexes and appropriate CO stretching frequencies 
are included in Figure 2-29. The next chapter discusses the reactivity of these adducts 






52 

with dihydrogen gas. Hopefully, the complexes discussed in this chapter will find uses in 
other bond activation reactions in the near future. 



Ph 



Me,Si 



N 



N Mo" Py 

icSi 




48vCO = 1913cm 



-l 



Ph 



Me.Si 



N 



N/ „ NI.,oPMe 3 
Mo 

\ 
Me 3 Si 



54vCO=1925cm 




i 



Ph 



Me.Si 



N 



Ph 



N 



301 N ^V" PMC2Ph ^^N- .Ji'-.^OMefe 

PhM^P* 7 I Vn (MeO) 3 P^ 

Me 3 Si \/ Me 3 Si 




\>, 



CO 



56vCO = 1921 cm 



-l 



51vCO=1959cm 



i 



Figure 2-29. Carbon monoxide complexes and vCO (cm" 1 ) 






CHAPTER 3 

SYNTHESIS AND REACTIVITY OF A MOLYBDENUM IMIDO-DIAMIDO 

STRETCHED DIHYDROGEN COMPLEX 

The first literature report to describe the isolation of a dihydrogen complex 
(referred to as r| 2 -H 2 or H 2 complexes) was put forth by Gregory J. Kubas in 1984. 74 
Since this date, over 350 stable H 2 complexes have been synthesized and characterized, 
and roughly 100 additional reported examples are either thermally unstable, transient 
species, or are proposed to contain H 2 ligands. Dihydrogen complexes of every metal 
from vanadium to platinum have been reported, and an example of a europium complex 
is known. Most complexes are coordinatively saturated and are cationic in nature. 
Several review articles have been written on the subject over the years _ 75 ' 76 ' 77,78 ' 79 ' 80 ' 81 
The most recent treatment of this area has taken the form of an excellent book written by 
Kubas. 82 

Dihydrogen complexes are important species in metal-mediated catalysis, as they 
represent "arrested" states along the path to dihydrogen bond-breaking or oxidative 
addition of the dihydrogen molecule. Metal-bound dihydrogen also displays unique 
chemical and physical properties, can be electrophilic or superacidic, and may exist in a 
stretched or an unstretched mode. 75 " 81 The unique properties displayed by dihydrogen 
upon binding to a metal center are a result of the metal-hydrogen bonding interaction. 

Bonding in H 2 complexes involves a-donation from the H-H a-bond to the metal 
and back bonding of electron density from a filled metal d orbital to the H-H a* orbital 
(Figure 3-1). The back bonding interaction is primarily responsible for elongating the 



53 



54 



/• 




H-H bond in an H2 complex. The oxidative addition process can be arrested at various 
stages through variation of the metal and ligands, producing complexes with different 
H-H distances (<4ih). In general, true H 2 complexes have d HH = 0.8 A-0.9 A and stretched 
(elongated) H 2 complexes have d HH = 1.0 A-1.6 A. In comparison, the d H u in free 
dihydrogen is 0.74 A while in metal dihydride species d HH > 1.6 A. 82 



H 
H 



a-donation back donation 

Figure 3-1 . General bonding in H 2 complexes 

Characterization of Dihydrogen Complexes 

Characterization of H 2 complexes is primarily carried out by measuring d m in 
related complexes. This value can be measured experimentally by diffraction methods, 
solid state NMR spectroscopy, and solution NMR spectroscopy. We will briefly discuss 
solution NMR methods here as it pertains to later discussions in this chapter. In general, 
d MU (metal-hydrogen distance) are not measured for several reasons. There are no 
general NMR spectroscopy techniques that can be used to measure this value. X-ray 
diffraction techniques produce large uncertainties in d MH and are of limited use. There 
are not enough d MH distances determined by neutron diffraction to draw useful 
correlations. Finally, d MH cannot be easily used to determine the degree of H-H 
activation in complexes with different metals because of differences in van der Waals 
radii. 



55 

Solution NMR Spectroscopy: </hh and '/h-d 

The single most important spectroscopic parameter involved in the characterization 
of an H 2 complex is "J H -d for the HD isotopomer of the H 2 complex. Classical dihydrides 
do not show any significant ' J H -d because there is no H-D bond present. In an H-D 
complex, there is still H-D bonding, and an H-D coupling constant can be measured by 
'H NMR spectroscopy. In general, the longer the H-D bond, the smaller the ' J H -d- The 
value of J H -d, determined by solution NMR spectroscopy, can be correlated to d m in the 
solid state by using the empirical relationships developed by Morris 83 and Heinekey 84 
(Equations 3-1 and 3-2, respectively). 

</hh=1.42-0.0167(./h-d)A [Morris] Equation 3-1 
d HH = 1.44- 0.0 168(J H -d) [Heinekey] Equation 3-2 
These relationships were created using d HH from neutron diffraction and solid-state NMR 
measurements for complexes where J H . D was known. A plot of d HH vs. l J H . D gave a 
straight line with little deviation. Therefore, if ' J H -d for any H-D complex is known, d HH 
can be calculated from the above relationships. The results obtained using Equations 3-1 
or 3-2 do not usually differ significantly. Equations 3-1 and 3-2 will be used to 
characterize H 2 complexes in this chapter to verify this statement. 
Solution NMR Spectroscopy: d HH and NMR Relaxation Time (Ti) 
Measuring the minimum value of the relaxation time (T Hmm) ) for the H nuclei of the H 2 
ligand can provide a reasonable estimate of d HH . The dipolar relaxation of one H of the 
H 2 ligand by its neighbor is the dominant contribution to T\ for H 2 complexes and is 
generally < 50 ms, whereas T x for hydrides is » 100ms. Furthermore, T, is proportional 
to d HH , and dipole-dipole relaxation theory states that T x varies with temperature and 
goes through a minimum at 7/ 1(min) . Crabtree and Hamilton have shown that at r I(min) , the 



56 

value ofd m can be determined. 85 ' 86 When using this approach, the researcher must be 
aware that contributions to relaxation from other sources, such as proton-containing 
ligands 87 !8 and metals with high gyromagnetic ratios (Co, Re, and Mn), 89 can complicate 
the interpretation of d H n from T l(min) . Another area of concern involves the effects of the 
rotational motion of the H 2 ligand on dm- This rotational motion is described as H 2 
ligand rotation being slower or faster than molecular tumbling, and can affect dipolar 
relaxation. Morris has addressed this and proposed two equations for the calculation of 
d H n from ri (min) , one for slow rotation of H 2 (Equation 3-3) and one for fast rotation of 
H 2 , where v is the spectrometer frequency (Equation 3-4). 90 Equation 3-3 will be used in 
the characterization of the H 2 complexes discussed in this manuscript. 
d HH - 5.81[Ti (m i n) /v] (1/6) Equation 3-3 (slow rotation) 
Jhh = 4.61 [r 1(min) /v] (1/6) Equation 3-4 (fast rotation) 
Molybdenum Imido-Diamido Stretched Dihydrogen Complexes 
The adducts presented in Chapter 2 were generated in order to explore their 
potential for the oxidative addition of small molecules. Our initial interest in this area has 
focused on the reactivity of these adducts with molecular hydrogen. Along these lines, 
we have synthesized phosphine-stabilized H 2 complexes from 52 and 55. 91 Of the 
various H 2 complexes known, none, to our knowledge, contain metal-ligand multiple 
bonds, and few contain amide functionalities. 92 Furthermore, these are rare examples of 
H 2 complexes in nominal d 2 configurations. Our initial studies concerning the synthesis 
and characterization of these unique H 2 complexes are reported herein. 









57 



Characterization of [Mo(NPh)(PMe 3 )2(H2)(o-(Me3SiN)2C 6 H 4 )] (60) 

Exposure of a cold (-10°C), toluene-^g solution of [Mo(NPh)(PMe3)3 
(o-(Me3SiN)2C6H4)] (52) to an atmosphere of molecular hydrogen (ca. 1 atm) resulted in 
a rapid color change from purple to green. The 'H and 3I P NMR spectra of this solution 
indicate that 52 and the H 2 complex [Mo(NPh)(PMe 3 )2(H2)(o-(Me3SiN)2C 6 H4)] (60) were 
present in solution in a 1 :3 ratio at -50°C (Figure 3-2). When allowed to warm to 30°C, 
60 underwent an additional transformation to give a purple solution of [Mo(NPh)(PMe3)3 
(o-(Me 3 SiN)(NH)C6H 4 )] (53) over a 1 h period. Thus, the net reaction of 52 with H 2 is 
addition of H 2 across the Si-N bond of the diamide ligand. This addition is, to our 
knowledge, unprecedented in the literature. 



Ph 

/ 

Me 3 Si v N 

N J]'-^ PMe 3 

Mo 



PMe, 




SiMe, 



Ph 



'MoCT 3 

MejP N h 

SiMe 3 




+ PMe 3 



H-Si- 



Me 3 Si v 

+ N ^PMe 3 

^PMc 



1 h (30 °C) 
+ PMe 3 




Figure 3-2. Generation and reactivity of H2 complex 60 

Dihydrogen complex 60 is stable for days under an atmosphere of molecular 
hydrogen (1 atm) at -20°C but will convert to 53 at this temperature over a period of 



58 

several weeks. Attempted isolation of 60 by concentration of toluene solutions in vacuo 
resulted in isolation of the starting material (52). Although efforts to scavenge PMe 3 with 
tris(pentafluorophenyl)borane 93 were successful, 60 remained reactive under these 
conditions, generating HSiMe3 and unidentified metal-containing products. Presumably 
lack of phosphine in solution compromises the formation of 53, and the (bis)phosphine 
analogue of 53 is not stable under these conditions. Complex 60 can be observed in 
degassed (H 2 free), phosphine-scavenged solutions by 'H NMR spectroscopy, but it 
decomposes rapidly. 

The conversion of 60 to 53 is shown as an irreversible step in Figure 3-2. This was 
confirmed by reaction of the deuterated analogue of 53 (53 D) with HSiMe 3 (Figure 3-3). 
At 20°C no H/D exchange was observed between the N-D and Si-H sites by 'H NMR 
spectroscopy, demonstrating that this step is irreversible. 

Ph 

/ 

Me 3 Si s N 

N jl"UPMe 3 \\y 

yf\\ + i * no H/D exchange 

y 'Mc^ PMe 3 £ 




53 D 

Figure 3-3. No H/D exchange between N-D and Si-H sites at 20°C 

A characteristic resonance in the *H NMR spectrum of 60 is a broad triplet at 3.59 
ppm ( 2 J P . H = 28 Hz, C 7 D 8 , -20°C) (Figure 3-4). The dramatically different chemical shift 
of these protons relative to the hydride protons of the tungsten analogue, [W(NPh) 
(PMe 3 ) 2 H 2 (o-(Me 3 SiN) 2 C 6 H4)], 94 (9.26 ppm, br, t, 2 J P . H = 40 Hz, 18°C, C 6 D 6 ) prompted 
us to investigate the metal-hydrogen interaction in more detail. 



59 

The H-D isotopomer of 60, 60D, was generated in order to determine d m from 
Jh-d as discussed above. The isotopomer (60D) displays both coupling with 
phosphorous (t, 1:2:1, V P . H - 28 Hz) and deuterium (t, 1:1:1, %. D = 15 Hz, C 7 D 8 , -20°C) 
(Figure 3-4). Using the %. D of 1 5 Hz in Equation 3-1 gives a value of 1 . 1 7 A for d m , 
which is typical of a stretched-H 2 complex. A comparable value of 1 . 19 A is calculated 
using Equation 3-2. 



3.59 




'''|'Mi|iin|iiii|i,,i|iiii|, l i l pnt|rHI|llll|llll|MII|I M I|ll ll|ill l | ll l l | ll l l|llll|llll||||||||||| M II| l lll |nM | M ll | l ll l| ll ll | llll|lll l | l lll|l||l|II M | 



3.72 3.68 3.64 3.60 3.56 3.52 3.48 



ppm 



27.8 



15.0 




60D 



(■'■■■■"■i'- J'" l l'<"J*''Miiii|iiii|iiii]iiii|iiii|irn| i ii mi i ij ii nmj i j iiii j iiii|iit j jiiii | i l n j ll l lt i I M p l ,,, f |, rr j t ,,, | ,,,,|,, 1 | |ll M . r ,,, [r , 

3.70 3.66 3.62 3.58 3.54 3.50 3.46 ppm 



Figure 3-4. Spectra (*H NMR) of the H 2 and H-D ligands of 60 and 60D (-20°C) 

The determination of d HH using a ri (min) analysis is consistent with d m as 
determined via 'j H . D . A plot of T\ vs. temperature for 60 yields a r I(min) of approximately 
36.99 ms, which is normal for an H 2 complex (Figure 3-5). Using Equation 3-3, a value 
of 1 . 1 9 A for d m is calculated. Due to the good agreement between d HH , as determined 



60 

by Jh-d, and T^mjn), the H2 ligand in 60 is most likely slowly rotating and is significantly 

stretched. 

In T\ vs. IC 1 



4.5 



In 7, 



3.5 



0.0037 0.0040 0.0041 0.0043 0.0047 0.0052 

K- 1 

Figure 3-5. Relaxation time vs. temperature plot for 60, In T\ vs. K" 1 

Two mechanisms can be proposed to account for the transformation of 60 to 53. 
These mechanisms will be presented separately. Both mechanisms will be presented with 
H-H(D) as the reactive molecule in order to represent reactions with H 2 and H-D gas (D 
will be written in bold for this discussion only). The D will always be placed in the 
nonbridging position, as is expected from zero point energy arguments. 95 In the first 
mechanism, HSiMe 3 is eliminated giving bent-imido 62 (Figure 3-6). The bent imido can 
be converted to 53 by H(D) migration and coordination of PMe 3 . The direct elimination 
of HSiMe 3 can proceed through the five-centered transition state 61. 

In the second mechanism (Figure 3-7), initial proton migration to an amide, 
through a four-centered transition state (63), gives rise to 64. Elimination of H(D)SiMe 3 
then takes place through another four-centered species (65) ultimately giving 53. 

When 52 is treated with deuterium hydride gas (H-D), an uneven distribution of H 
and D is observed in the products, as shown in Figure 3-8. The H atoms prefer the N site 



61 



of 53 and the D atoms prefer the Si site of SiMe3. This H/D distribution corresponds to 
an H/D isotope effect of approximately 2.0. The mechanism shown in Figure 3-7 is the 
most consistent with this isotope effect, as it places more H at the N site while the 
mechanism in Figure 3-6 does the opposite. These mechanisms assume that the rate 
determining steps involve the initial proton transfers (61 to 62 in Figure 3-6 and 63 to 64 
in Figure 3-7) and that all other steps are irreversible. 



Me,Si 




^PMei 



%H(D) 

(Me^ H 

\ 
SiMe 3 

60 



Ph 



Me 3 Si 



N 



>xPMe 3 



, N mo:: 

J\ " H J D ) 

,-H 




61 



Ph 



Me.Si 



N 



N ^- PMe 3 



;Me,P"xT PMe 3 




Ph 



Me 3 Si 



N 



N".-JiL' oPMe 3 



"" Mol 




H(D) 



+ 
H-SiMd 



+PMe 3 
H migration 



Figure 3-6. Potential mechanism for the formation of 53 

The mechanism presented in Figure 3-7 is also favored over that in Figure 3-6 
based on the known reactivity of dihydrogen complexes. The H 2 ligand protons are 
known to become acidic once coordinated to a metal center (pAT a ' s of 1 5-20 for neutral 
complexes). The H-H(D) ligand in 60 could protonate an amido ligand giving rise to 
64. Proton transfer from bound H 2 to nitrogen-containing ancillary ligands has been 












62 



proposed in the literature. 92 ' 97 Elimination of H(D)SiMe3 can then occur as shown in 
Figure 3-7. The mechanism shown in Figure 3-6 involves the formation of the 
bent-imido species 62. Although bent imides are known, this species should be of high 
energy and thus it is not very favorable. 



Ph 



Me 3 Si s N 



""Mo 




^PMe, 



V*(D) 

e 3? N H 

SiMe 3 



60 



Ph 



Me.Si 



N 



N/, ilUPMe 3 

Mo. 




Ph 



Me.Si 



N 




[e/| H(D) 



/ H 



Me.Si 



64 



Ph 



Me.Si 



N 






N„,, iii.,oPMe 3 H(D)SiMe 3 elimination 
\ + PMe 3 

,P,XT PMe 3 




Me.Si 



Ph 



N 



Mo'^ PMC3 

Me/ >(»> 




N--. 



SiMe. 



Figure 3-7. Potential mechanism for the formation of 53 

Ph 



VI 


e 3 Si 


N 
N Mo'° PMe3 


-\\ 


/Me/^ PMe 3 

"/r^ \ 

H(D) 






53 






2H: ID 



(D)H— SiMe 3 



1H:2D 
Figure 3-8. H and D do not partition equally between N and Si sites 



63 

Characterization of [Mo(NPh)(PMe 2 Ph)2(H2)(o-(Me3SiN)2C 6 H4)l (66) 

Complex 55 reacted with dihydrogen gas to give [Mo(NPh)(PMe 2 Ph)2(H 2 ) 
(o-(Me 3 SiN) 2 C 6 H4)] (66) (Figure 3-9). At 20°C, 66 converted to 67 with loss of HSiMe 3 . 
This reactivity is similar to that observed with H 2 complex 60. The main difference 
between the formation of 60 and 66 involves the phosphine starting materials. The 
formation of 60 from 52 involves initial loss of PMe3, and at -50°C an equilibrium 
between 52 and 60 exists. Generation of 66 does not require loss of phosphine from 55. 
as 55 is already coordinatively unsaturated. Furthermore, in reactions where 55 was 
treated with H 2 , 66 was observed as the only metal species in solution by 'H and 31 P 
NMR spectroscopy. 

The expanded region of the H 2 and H-D ligand resonances in the 'H NMR spectra 
of 66 and its H-D isotopomer, 66D, are shown in Figure 3-10 (C 7 D 8 , -20°C). The H 2 
ligand resonance for 66 appears as a very broad triplet centered at 3.46 ppm in the 'H 
NMR spectrum. A broad, five-line resonance is observed for the H-D ligand of 66D, 
centered at 3.46 ppm. The isotopomer (66D) displays both coupling to phosphorous (t, 
1:2:1, 2 y P . H - 21 Hz) and deuterium (t, 1:1:1, 'J H -d = 21 Hz, C 7 D 8 , -20°C) (Figure 3-10). 
The identical 2 J P . H and 'J H -d for 66D produce the 1:3:4:3:1 multiplet observed in the 'H 
NMR spectrum. Using Equation 3-1, a d H H value of 1.07 A is found for 66, and a value 
of 1.09 A is calculated using Equation 3-2. 

A Tifam) analysis of 66 gives a d m value of 1 .10 A (Figure 3-11). This value is in 
excellent agreement with the value found through use of 'Jh-d, indicating that the H 2 
ligand is slowly rotating. 



64 



Ph 



Me.Si 




N 

\ JN/PMe 2 Ph H 2 
N— Mo 

r/ X PMe 2 Ph 



SiMe 3 
55 



Ph 



Me.Si N 



'3' 31 v 



,vNPMe,Ph 



/ „Mo^ o 
PhMfe 2 P / V* 




Ph 



Me 3 Si N 

J N„„ N!,,PMe 2 Ph V 
Mo + si 




PMe.Ph 



/|\ 



\ 



SiMe, 



66 



Figure 3-9. Generation and reactivity of H 2 complex 66 




1 1' 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 

3.75 3.65 3.55 3.45 3.35 3.25 ppm 



3.46 



20.9 



20.9 



66D 




1 1' '' I ' I' 1 1 1 1 1 1 1 1 1 1 1 [ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 [ 1 1 1 1 1 



i 1 1 1 i 1 1 



3.75 3.65 3.55 3.45 3.35 3.25 ppm 



Figure 3-10. Spectra ('H NMR) of the H 2 and H-D ligands of 66 and 66D (-20°C) 



65 



In 7i vs. K" 



lnT, 



3.7 
3.6 
3.5 
3.4 
3.3 
3.2 
3.1 

3 J 

0.0047 0.0044 0.0041 0.0039 0.0037 0.0036 0.0033 

K" 1 
Figure 3-11. Relaxation time vs. temperature plot for 66, In T\ vs. K" 1 
Bonding in Molybdenum Imido-Diamido Stretched Dihydrogen Complexes 

A general bonding scenario for H 2 complexes 60 and 66 is shown in Figure 3-12. 

The 7i MOs involved with metal-imido anti-bonding are shown at the highest energy in 
Figure 3-12. The bonding combinations of these MOs are much lower in energy. The 
metal-H 2 interaction is shown at an intermediate energy. This interaction involves back 
bonding from the metal d Ky atomic orbital to the H 2 a* atomic orbital. The back bonding 
interaction is primarily responsible for lengthening the H-H bond due to population of the 
H 2 a atomic orbital. If this is true, increasing back bonding by increasing the electron 
density at the metal center should lengthen the H-H bond. 

The d HH of 1 . 1 9 A for 60 is longer than the d m of 1 .09 A for 66 (using Equation 
3-2). This difference in bond length reflects the difference in electron density at the 
metal center. Since PMe 2 Ph is less electron donating than PMe 3 , d m in 66 should be 
shorter than d UH in 60, as there is less back bonding to the H 2 a* orbital in 66. 



66 



/ 



Me 3 Si 



>( vPMe 2 X 




V 



NX 



III 71* dxz, dyz 
Mo 



^^fl fr dxy + H 2 CT * 



e 



K 



M=N ^-bonding 



60, X = Me 
66, X = Ph 



Figure 3-12. Bonding scenario for H2 complexes 60 and 66 
Reaction of 50 and 57 with Dihydrogen Gas 

When samples of 50 and 57 were treated with H 2 gas in NMR tubes (C 6 D 6 ) no 
initial change in the sample composition was observed by 'H NMR spectroscopy. The 
samples did react with H 2 gas over 1 week, in each case generating HSiMe 3 and metal 
complex (Figure 3-13). We have not fully characterized the metal-containing species 68 
or 69 but have assigned tentative structures because of the similarities in *H NMR spectra 
with 53 and 67. Presumably, the less electron donating phosphite ligands in 50 hinder 
formation of significant amounts of H 2 complex in solution when 50 is treated with H 2 
gas. We propose that small concentrations of H 2 complex exist in solution, allowing for 
formation of HSiMe 3 and 68 after one week of reaction time. 

Small concentrations of H 2 complex must also be present in solution when 57 is 
treated with H 2 gas in order to account for the formation of HSiMe 3 . The cis 
coordination of the DMPE ligand may inhibit the large-scale formation of a species with 






67 



trans phosphines capable of coordinating H 2 . A possible H 2 complex of this system 
could be the "arm off complex (70) shown in Figure 3-14. The chelate effect of DMPE 
would certainly curtail the formation of large amounts of 70. 



Ph 



Ph 



Me 3 Si, 



N 



N^UP(OMe) 3 H 2 (ca.l5psi) N NUP(OMe) 3 

I V™, * HSiMe 3 + (MeO) 3 P-fj\ 

_jl P(OMe) 3 J\-h P ( 0Me >3 

x / // \ 

Me 3 Si \/ h 




50 



Me 3 Si 




SiMe, 



Ph 



J&aZ* /" H 2 (ca. 15psi) 
^111 ^p— v \ / N \/ — i = =-*- HSiMe 3 + 

*\ A ^"Z-^ 



57 




// SiMe 3 






68 




69 



Figure 3-13. Reaction of 50 and 57 with Dihydrogen Gas 




Me.Si 



- P "".4)- N ' 



SiMe, 



Ph 



/\ 



'^» 



N 



p—\ \/ N H 

Ph Me,S,YlV 
SiMe, 




~PMe, 



Figure 3-14. Possible H 2 complex in the DMPE system 



68 

Reaction of 52 with Phenylsilane and Diphenylsilane 

Complex 52 reacted with phenylsilane and diphenylsilane in C 6 D 6 . The major 
products of these reactions at 20°C are cyclic silyl amines 71 and 72 (Figure 3-15). The 
metal-containing products in these reactions have not yet been identified. The structures 
of compounds 71 and 72 have been tentatively assigned using the following data. Cyclic 
71 has been isolated and characterized by 'H NMR and mass spectroscopy. We propose 
that 72 exists based on *H NMR data only. Further experiments to positively identify 
these materials, especially 72, will be required. 

When 52 reacted with H 3 SiPh at low temperature (-30°C), 72 formed along with a 
substantial amount of the dihydrogen complex 60. Complex 60 decomposed to 53 and 
HSiMe 3 when left to stand at 20°C. The exact nature of the mechanism that generates 60 
in this fashion is not known. A dehydrocoupling pathway does not seem to be active, as 
the expected silicon-based dehydrocoupling products (for example: PhSi(H) 2 Si(H) 2 Ph) 
are not observed by 'H NMR. 96 

Presumably, thermodynamics drives the formation of 72 when 52 is treated with 
H 3 SiPh at 20°C. In contrast, low temperatures favor the formation of 60, the kinetic 
product of this reaction sequence. Very small amounts of HSiMe 3 and 53 are observed in 
the reaction mixture after treatment of 52 with H 2 SiPh 2 at 20°C. This reaction must be 
explored at lower temperature in the future. 
Summary 

Reactions of molybdenum imido-diamido phosphine adducts of PMe 3 (52) and 
PMe 2 Ph (55) with CO gas were discussed in Chapter 2. In the case of 52 CO displaces 
PMe 3 to give a carbonyl complex, whereas CO coordinates to coordinatively unsaturated 
55 to give the appropriate metal carbonyl. In this chapter we report that the phosphine 



69 






complexes 52 and 55 react with dihydrogen gas to give stretched dihydrogen complexes 
60 and 66, respectively. The formation of 60 from 52 is interesting in that a PMe 3 ligand 
is displaced from the metal center by a much weaker H? ligand. Steric crowding around 
the metal center may help influence PMe 3 dissociation from 52, allowing for the 
generation of a (bis)phosphine complex that can coordinate H 2 . Displacement of 
phosphine by H 2 is a rare process, and to our knowledge, only one other example has 
been reported in the literature. 82 

SiMe, 



Me.Si 




H,Si 



2 oi x 



Ph 
*Ph 



^FMe 3 

S PMe, 



SiMe, 



/^^N^ /Ph 

\ 
SiMe 3 

71 



SiMe 3 



H 3 Si-ph 




Figure 3-15. Generation of cyclic 71 and 72 

We also discuss the reactivity of the dihydrogen complexes 60 and 66. These 
reactions proceed by formal addition of H 2 across the N-Si bond, liberating HSiMe 3 and 
giving metal complexes 53 and 67. While proton transfer from H 2 ligands to amido and 
amine ligands has been proposed in the literature 92 ' 97 and may play a role in the 
mechanism of this reaction, elimination of H-SiMe 3 from a metal center in this way is, to 
our knowledge, unprecedented. 



70 

Preliminary results concerning the reaction of 52 with H2SiPh 2 and H 3 SiPh are 
reported here. Future efforts toward characterization of several products are required. In 
the interim, we propose that thermodynamics drives the formation of 71 and 72 at 20°C. 
The dihydrogen complex formed when 52 and H 3 SiPh react at low temperature is most 
likely a kinetic product of the system. 
























CHAPTER 4 

SYNTHESIS, CHARACTERIZATION, AND REACTIVITY OF A MOLYBDENUM 

(IV), r] 4 -BUTADIENE COMPLEX AND n. 2 -ALKYNE COMPLEXES 

Early Transition Metal Butadiene Complexes 

Low valent zirconium (0) butadiene complexes with bidentate phosphine ligands 
were among the first well-characterized examples of Group 4 metal butadiene 
complexes. The well-known, higher valent, base-free zirconocene butadiene complexes 
were independently synthesized by both Erker" and Nakamura 100 (Figure 4-1). Among 
the striking structural characteristics of these complexes is the ability of the butadiene 
fragment to coordinate to the metallocene core in eft and trans-modes and the dynamic 
envelope shift isomerization process associated with the eft-coordination mode. 99 " 101 

The structure and bonding of eft-butadiene complexes can vary between two 
extremes, the n 2 and the a 2 , n-designations (Figure 4-2). 95100 The bonding in n 2 
complexes is best described as donation of butadiene 7r-electron density to the metal 
center. A number of n 2 complexes have been characterized by X-ray crystallography, 
and in these compounds the butadiene C-C bond lengths are very similar. In a 2 , n-type 
complexes, the butadiene ligand is considered a dianionic dialkyl, a result of considerable 
back bonding. Solid-state structural studies have shown that the C(l)-C(2) and C(3)-C(4) 
bond lengths are considerably longer than the C(2)-C(3) bond in a 2 , 7i-complexes (Figure 
4-2). The substitution of the butadiene ligand and the identity and electronic structure of 
the metal play a large role in dictating which structure type will be adopted. To date 
there are several examples of structurally characterized Group 4 and 5 eft 101 and 



71 



72 



102 



trans -butadiene metal complexes, with the cisoid conformation being the most 



common. 











Zr 



• >>v> 



^ 



C(l)^ ^C(4) 
M 

2 



Erker et al. 



Figure 4- 1 . Synthesis of zirconocene butadiene complexes 

, , C (2), A3) 

^> c 

M 

Figure 4-2. Possible 7r and ct 2 , n structures for cw-butadiene complexes 

Characteristic reactivity of Group 4 and 5 butadiene complexes involves reactions 
with unsaturated substrates such as carbonyl compounds, nitriles, and alkynes. 103 This 
reactivity is typified by Cp 2 Zr(butadiene) where C-C coupling of the unsaturated 
substrate with a terminal butadiene carbon results in metallacyclic complexes (Figure 
4-3). In some cases, the interconversion of these metallacyclic compounds can be 
observed experimentally. The metallacycles are generally stable, and hydrolysis 
protocols are required to free useful acyclic organic products from the metal center. In 
sharp contrast, later transition metals (Groups 9 and 10) catalyze the intermolecular 4+2 
cycloaddition of nonactivated substrates, 104 while related transition metal-catalyzed 



73 

cycloisomerization reactions constitute a rapidly developing area of research. 105 Recent 
developments involving Group 4 butadiene complexes involve their applications as 
catalysts in olefin polymerization reactions. 106 



Zr(Cp) ; 



+ A=B 



(Cp) 2 Zr 




* (Cp) 2 Zr. 



V B 



(Cp) 2 Zr 




A-B 



H I J 

Figure 4-3 . Reactivity of Cp 2 Zr(butadiene) with a representative unsaturated substrate 

Reports concerning the synthesis and characterization of high valent Group 6 
transition metal butadiene complexes are not as common as those of the earlier Groups. 107 
Furthermore, reactivity of these Group 6 butadiene complexes differs from the 
well-explored reaction chemistry associated with the Group 4 and 5 butadiene complexes 
and the catalytic cycloisomerization reactions of the later metals. 107 

Molybdenum Imido-Diamido Butadiene Complexes 
We have recently prepared a butadiene complex of our molybdenum 
imido-diamido system. The synthesis, structural characterization, and reactivity of this 
monomeric, diamagnetic, molybdenum (IV)-cw-butadiene complex, 
[(Mo(NPh)-n 4 -(H 2 C=CHCH=CH 2 )( -(Me 3 SiN) 2 C 6 H4)] (73) is discussed below. 108 To 
our knowledge, this is the first report of a molybdenum complex of this nature. While 
structurally similar to other molybdenum butadiene complexes, the reactivity of 73 
departs from these showing similarities to both early and late transition metal reactivity. 









74 

The synthesis of a related imido-diamido methylvinyl ketone (MVK) complex is also 
discussed here. 

Synthesis and Characterization of r| 4 -Butadiene Complex 
[(Mo(NPh)-Ti 4 -(H 2 C=CHCH=CH2)(o-(Me3SiN) 2 C 6 H 4 )](73) 

Treatment of a pentane solution of 37 or 38 with molecular butadiene gave the 
r| 4 -cw-butadiene complex [(Mo(NPh)-r| 4 -(H 2 C=CHCH=CH2)(o-(Me3SiN)2C 6 H4)] (73) in 
good yield (Figure 4-4). Decomposition of 73 occurred within 4 h at 20°C, affording an 
intractable mixture, as determined by 'H NMR spectroscopy. When kept at -30 °C in the 
solid-state, 73 did not decompose appreciably over several months, as noted by ' H NMR 
spectroscopy. An X-ray crystallographic study was carried out on a single crystal of 73 
grown from a -30°C solution of pentane/methylene chloride. The crystal data and details 
of the structure refinement are summarized in Table 4-1. 



Me.Si. 




Ph 

/ 

IN Rl a R < R 2 

'N— Mo v L R Butadiene gas Me,Si^ XT -¥ v 'y/\ V 

Me 3 Si7-N ^/r K 2 . 3 N S/\ + 

fy U A Pentane RT MesSi^-N J/ )\ 

II H JN H H 

R, = Me,R 2 = H(37), 
R, = R 2 = Me (38) 

Figure 4-4. Synthesis of [(Mo(NPh)-Ti 4 -(H 2 C-CHCH=CH 2 )(o-(Me3SiN)2C 6 H4)] (73) 

The butadiene complex crystallizes in a monoclinic unit cell with one molecule of 
methylene chloride. The molecular structure of 73, accompanied by selected bond 
lengths and angles, is shown in Figure 4-5. The butadiene fragment clearly adopts a 
cw-arrangement when bound to the metal center. The metal to terminal butadiene carbon 
distances of 2.254(3) A (Mo-C(19)) and 2.257(2) A (Mo-C(22)) and the Mo-C(20) and 


















































75 



_2 



Mo-C(21) bond lengths of 2.336(2) A and 2.355(2) A, respectively, support a tt 2 
r\ -butadiene bonding motif for 73. Other molybdenum butadiene complexes adopt 
similar bonding modes. 107 Within the butadiene fragment, the similar C(19)-C(20), 
C(20)-C(21), and C(21)-C(22) bond lengths of 1.401(4) A, 1.397(4) A, and 1.405(4) A, 
respectively, also support a tt 2 , n 4 -bonding mode for 73. The Mo-N(l) length of 
1 .75 1 7(19) A is typical of a Mo-N triple-bond interaction and is comparable to the 
Mo-N(l) lengths in similar complexes (see Chapter 2). 11,27 - 47 

It is common practice to use the ' J C -h coupling constants to determine the relative 
degree of sp~- sp 3 hybridization of the coordinated diene carbons in metal butadiene 
complexes. By determining the relative degree of hybridization of the diene carbon 
atoms, the structure, in terms of butadiene bonding, can be assigned a position 
somewhere between the two extremes n 2 or a 2 , n. Using Newton's semi-empirical 
rule, it is possible to calculate the % $ character of carbon atoms in the dienes and 
hence the hybridization. The value of n for the carbons at diene termini reaches 2.8-2.9 
(132-128 Hz) when the molecule adopts a a 2 , 7i-type structure while the value is in the 
range of 2.1-2.3 (165-154 Hz) in the case of a ^-complex. 1 10 We have assigned the 
observed triplet in the 13 C NMR spectrum of 73 at 75.0 ppm to the terminal butadiene 
carbons, and the observed 'j C -H 158 Hz coupling constant is in good agreement with the 
formulation of 73 as a 7i 2 -butadiene complex. In light of these results regarding 73, the 
formal oxidation state at the metal center is best represented by Mo(IV). 

Analogous tungsten imido-diamido complexes have been prepared in our 
laboratories. ' ' These complexes are better described as adopting the a 2 , n-butadiene 
bonding mode. This trend has been observed for Group 4 butadiene species where 



76 



zirconium complexes prefer 7r 2 -type bonding, while hafnium complexes prefer a 2 , n type 
bonding. This is as expected and follows the general trend that metal-carbon a-bonds of 
third row transition metal complexes are stronger than the corresponding bonds in the 
isostructural second row complexes. I00(b) ' 112 




Figure 4-5. Thermal ellipsoid plot of 73 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.7517(19), Mo-N(2) 2.0556(17) 
Mo-N(3) 2.0536(18), Mo-C(19) 2.254(3), Mo-C(20) 2.336(2), Mo-C(21) 
2.355(2), Mo-C(22) 2.257(2), C(19)-C(20) 1.401(4), C(20)-C(21) 1.397(4) 
C(21)-C(22) 1.405(4), C(19)-C(20)-C(21) 124.4(2), C(20)-C(21)-C(22) 
123.3(2), C(l)-N(l)-Mo 147.27(17), N(3)-Mo-N(2) 78.24(7) 



77 



Table 4-1. X-ray data 3 for crystal structures 73, 74, and 78 





73 


74 


78 


Chemical formula 


C 2 2H33N3MoSi2-CH 2 Cl2 


C 2 6H39N 3 MoSi2 


C22H 3 3N 3 MoOSi2 


Formula weight 


576.56 


545.72 


507.63 


Crystal system 


Monoclinic 


Monoclinic 


Monoclinic 


Space group 


P2i/n 


P2,/c 


P2,/n 


li(Mo-Ka) (mm" 1 ) 


0.775 


0.590 


0.641 


a (A) 


11.9736(6) 


16.031(1) 


10.0648(5) 


*(A) 


15.8844(8) 


10.0300(8) 


17.0457(9) 


c(k) 


14.5066(7) 


16.906(1) 


14.4866(8) 


m 


95.324(1) 


93.351(1) 


90.371(1) 


K (A 3 ) 


2747.2(2) 


2713.7(4) 


2485.3(2) 


z 


4 


4 


4 


'-'max 


27.50 


27.50 


27.50 


Total reflections 


16414 


19246 


16938 


Uniq. reflections 


6282 


6216 


5672 


R(int) 


0.0434 


0.0638 


0.0250 


Ri [/>2a(7)data] b 


0.0307 


0.0356 


0.0228 


wR 2 (all data) c 


0.0815 


0.0947 


0.0616 


Larg. diff. peak, hoi 


i 0.365, -0.425 


0.582, -0.672 


0.339, -0.291 



a Obtained with monochromatic Mo Ka radiation (k = 0.71073 A) at 173 K b /?, = sIIf I 
Fj/slFj. c w/? 2 = {l[w(F 2 - F c 2 ) 2 /Z[w(F 2 ) 2 ]} 1 ' 2 . 



78 

Reactivity of [(Mo(NPh)-r) 4 -(H 2 C=CHCH=CH 2 )(0-(Me3SiN) 2 C 6 H4)] (73) with 
2-Butyne 

The reactivity of 73 with 1.0 or 2.0 equiv of 2-butyne was explored. Proton NMR 
spectra of these reaction mixtures revealed the formation of two intermediates that 
disappeared within 30 min at 20°C, giving rise to the final products, as outlined in Figure 
4-6. Reaction of 73 with 1.0 equiv of 2-butyne afforded the molybdenum 
2,3-dimethyl-l,3-cyclohexadiene complex (74), as determined by *H NMR spectroscopy, 
while reaction with 2.0 equiv of 2-butyne gave l,2-dimethyl-l,4-cyclohexadiene and the 
molybdenum n. 2 -alkyne complex, [(Mo(NPh)-r) 2 -(MeCCMe)(o-(Me3SiN)2C 6 H4)] (75), 
according to 'H NMR spectroscopy (this alkyne complex has been synthesized 
independently, vide infra). Complex 74 was prepared independently by treatment of 38 
with excess l,2-dimethyl-l,4-cyclohexadiene (Figure 4-6). In addition, the 
2,3-dimethyl-l,3-cyclohexadiene ligand in 74 was not displaced by 2-butyne at room 
temperature, as observed by *H NMR spectroscopy. 

The identity of 74 was confirmed by X-ray crystal structure analysis, and the 
thermal ellipsoid plot is shown in Figure 4-7. The crystal data and details of the structure 
refinement are summarized in Table 4-1 . In 74 the metal is bound to the 
2,3-dimethyl-l,3-cyclohexadiene in an r| 4 -mode reminiscent of 73. The C-C distances 
between C(21)-C(20), C(20)-C(19), and C(19)-C(24) in 74 (1.418(3) A, 1.418(3) A, and 
1.405(3) A respectively), as well as the metal-carbon bond lengths, are similar to the 
corresponding distances in 73. The Mo-N(l ) distance of 1 .7686(1 8) A is consistent with 
a molybdenum nitrogen triple-bond interaction. The Mo-N(2) and N(3) amide distances 
of 2.0561(18) A and 2.0492(17) A, respectively, are within the range expected for Mo-N 
single bonds. 27 '- 91 Complex 74 is stable in solution at 20°C for weeks. We attribute 



79 



the difference in stabilities between 73 and 74 in part to steric crowding around the metal 
center in 74. 



Me 3 Si^ N — N 
Me 3 Si7— N 




Me 3 Sk 

Me 3 Sif^N 



Figure 4-6. Reactivity of [(Mo(NPh)-r| 4 -(H 2 C=CHCH=CH2)(o-(Me 3 SiN)2C 6 H4)] (73) 
with 2-butyne 

When 73 was treated with 1.0 equiv of 2-butyne at -20 °C, the accumulation of two 

metal-containing species was observed by ! H NMR spectroscopy over 24 h. These 

species are the fleeting intermediates observed during this reaction at 20°C and are stable 

at -20 °C for extended periods (over 2 days). An array of two-dimensional NMR 

spectroscopic techniques was used to elucidate the structures of these intermediates. The 

intermediates consist of two isomeric complexes present in a 4:1 ratio. The major 









80 



intermediate has been characterized as [^«-(Mo(NPh)(C(Me)=C(Me)CH 2 CHCHCH 2 ) 
(o-(Me 3 SiN)2C 6 H4)] (76a), a 5y«-r] 3 -allyl metallacyclic system. 



C3 




C26 



C25 



Figure 4-7. Thermal ellipsoid plot of 74 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.7686(18), Mo-N(2) 2.0561(18) 
Mo-N(3) 2.0492(17), Mo-C(21) 2.258(2), Mo-C(20) 2.342(2), Mo-C(19) 
2.402 (2), Mo-C(24) 2.274(2), C(21)-C(20) 1.418(3), C(20)-C(19) 1.418(3) 
C(24)-C(19) 1.405(3), C(24)-C(23) 1.518(3), C(23)-C(22) 1.542(3), 
C(22)-C(21) 1.525(3), C(l)-N(l)-Mo 142.47(16). 

The results of the structural elucidation study are presented in Figure 4-8. The 
metallacycle H a -C and H b -C l J c . H of ca, 160 Hz are consistent with an sp 2 hybridization 
of the carbon resonance at 76.8 ppm. This along with the lack of resolved coupling 
between H a and H b , support the existence of an r| 3 -allyl functionality. The chemical 



81 

shifts of the remaining allyl proton and carbon nuclei also support the allyl fragment in 
76a. The sequence of the protons which display multiplets was seen in the DQCOSY 
spectrum. NOe's between the methyl at 1.58 ppm and the methylene protons at 2.59 ppm 
and 2.67 ppm identified it as the one proximal to the methylene group. The protonated 
carbons were assigned to the corresponding protons using a GHMQC spectrum. The 
GHMBC spectrum displayed long-range couplings between the quaternary carbons at 
177.5 ppm and 154.7 ppm and the protons of both the methylene and the methyl groups, 
which confirm the metallacycle fragment. The relative sizes of the nOe's place H b , H d , 
and H e on one side of the ring and H a , H c , and H f on the other side. In 76a H a , H c , and H f 
showed nOe's to aromatic proton H g . We state that H g is an ortho phenyl imido proton 
and not a phenylene proton because in the DQCOSY spectrum they displayed a phenyl 
coupling pattern (7.15 ppm (H g ) - 6.98 ppm - 6.79 ppm). Therefore H c occupies a syn 
relationship with respect to the phenyl imido group. This intermediate metallacyclic 
7r-allyl system most likely arises from C-C coupling at the conjugated diene terminus. 
The minor species also consists of a metallacyclic 7i-allyl system, 

[fl«//-(Mo(NPh)(C(Me)=C(Me)CH 2 CHCHCH 2 )( -(Me 3 SiN) 2 C 6 H4)] (76b), that is very 
similar to 76a (Figure 4-9). Metallacycle 76b differs from 76a in that in 76b the central 
allyl proton H c is anti with respect to the phenyl imido group as revealed by the nOe's 
between H g at 7.19 ppm and H a , H d , and Hf. The details of the structural elucidation of 
76b are similar to those of 76a, however the low concentration of 76b in solution 
prevented the assignment of the 13 C chemical shifts of the metallacycle fragment from the 
GHMQC and GHMBC spectra. The NOESY spectrum did not display any chemical 
exchange peaks between 76a and 76b at -20 °C. 



82 



177.5, 154.7 




Figure 4-8. Structure of intermediate (76a), showing selected carbon (underlined) and 
proton chemical shifts, assigned by NMR spectroscopy. The diamido 
ligand has been omitted for clarity. 




Figure 4-9. Structure of intermediate (76b), showing selected proton chemical shifts, 
assigned by NMR spectroscopy. The diamido ligand has been omitted for 
clarity. 

There has been extensive mechanistic work by Erker et al. and Nakamura et al. on 

the reactivity of Group 4 metallocene derivatives of 1,3 -butadienes with unsaturated 

substrates. The pathway favored for such reactions involves coupling of the unsaturated 

moiety with one of the diene double bonds, producing a 2-vinylmetallacyclopentane 

species (H) (Figure 4-3). This species can then isomerize as shown in Figure 4-3. 



83 



It thus seems reasonable to propose that 73 reacts with 2-butyne to give 76e, which 
does not accumulate to any detectable levels by 'H NMR spectroscopy (Figure 4-10). 
Intermediate 76e rapidly rearranges, affording 76a and 76b, which can be observed at 
low temperature. At 20°C 76a, 76b, or some combination of the two most likely 
rearrange to form 76f, which rapidly generates 76g via reductive elimination. 
Isomerization of 76g ultimately results in the formation of 74. The production of 74 from 
treatment of olefin complex 38 with excess l,2-dimethyl-l,4-cyclohexadiene (Figure 4-6) 
gives evidence supporting the formation of 76g in the proposed mechanism. 




74 



75 



Figure 4-10. Proposed mechanism for reaction of 73 with 2-butyne. Ancillary ligands 
have been omitted for clarity. 

The difference in product distribution when 2.0 equiv of 2-butyne reacts with 73 is 

consistent with the proposed mechanism in Figure 4-10. Excess 2-butyne competes for 

the metal center with the l,2-dimethyl-l,4-cyclohexadiene ligand in 76g, liberating free 





















84 

1,2-dimethyl-l ,4-cyclohexadiene and forming 75 as the metal-containing product. In the 
absence of excess 2-butyne isomerization of 76g to 74 occurs. 

Reactivity of [(Mo(NPh)-n 4 -(H 2 C=CHCH=CH 2 )(o-(Me3SiN) 2 C 6 H 4 )] (73) with 
Acetone: Formation of [(Mo(NPh)(CH2CH=CHCH 2 C(Me)20)(o-(Me3SiN)2C 6 H4)] 

(77) 

When a green C 6 D 6 solution of 73 was treated with 1 .0 equiv of acetone, a red 
solution containing 77 was generated (Figure 4-11). Organometallic, metallacycle 77 
was characterized by an array of NMR spectroscopic techniques, including GHMBC, 
GHMQC, TOCSY, NOESY, and COSY. The results of the structural elucidation are 
shown in Figure 4-12. 

Ph Ph 



Me.Si 



N 

III 
Mo., 

N' / 
Me 3 Si,—N 



73 



N 

1.0 Acetone N/fp -• Mo 

m Me 3 bi^ N - i 




C 6 D 6 RT 





Figure 4-11. Formation of [(MorNPh)(CH 2 CH=CHCH2C(Me)20Xo-(Me3SiN) 2 C 6 H4)l 
(77) 




1.08,27.4 



0.41, L8Me 3 Si 

\ ,-Ma 

N V V 
0.42, 2.7Me 3 Siy-N 




/ 
3.62,708 \5.78, 134.1 



1.96,2.11,44.2 
4.57, 130.2 



Figure 4-12. Proposed structure for 77 showing selected carbon (underlined) and proton 
chemical shifts, assigned by NMR spectroscopy 












85 

Exchange peaks are observed between the methyl groups at 1 .08 ppm and 1 .37 ppm 
in the NOES Y spectrum of 77 at 25°C, corresponding to a dynamic process that 
exchanges these methyl group environments. Furthermore, at 25°C there are two 
resonances corresponding to inequivalent SiMe 3 groups in the 'H NMR spectrum of 77 at 
0.41 ppm and 0.42 ppm. At 60°C these resonances have coalesced, and the AG J for this 
process is approximately 74.37 KJ/mol (17.77 Kcal/mol), using Equation 2-1. 

The epimerization process shown in Figure 4-13 could account for this behavior. 

In Figure 4-13 the square pyramidal enantiomers, 77a and 77d, can interconvert through 

a Berry pseudorotation mechanism involving the trigonal bipyramidal intermediates 77b 

and 77c. This process clearly exchanges the methyl groups labeled Me a and Me b in 

Figure 4-13. At 25°C the epimerization process could be slow enough on the NMR time 

scale as to allow observation of two SiMe 3 resonances in the 'H NMR spectrum. If at 

60°C the epimerization process is fast on the NMR timescale, then only one SiMe 3 

resonance will be observed by 'H NMR spectroscopy. Different dynamic NMR 

properties have been observed for similar early-metal complexes. I02(c) 

Synthesis and Characterization of r| 4 -MVK Complex [(Mo(NPh)-r) 4 - 
(0=C(Me)CH=CH 2 )(o-(Me 3 SiN) 2 C6H4)](78) 

The coordination chemistry of Group 6 metals with acyclic conjugated dienes other 
than butadiene, more specifically a, p-unsaturated carbonyl compounds, is a topic under 
current investigation in the scientific comunity. Recent developments in this area involve 
the synthesis of a tris methylacrylate molybdenum complex, [Mo(H 2 C=CHCOOMe) 3 
(PMe 3 )], where two methylacrylate groups bind to the metal through both C=C and C=0 
functionalities as n 4 -l-oxa-l,3-diene ligands while the third associates with the metal 
through the C=C double bond." 3 A series of related compounds was reported by 



86 



Schmidt et a/., 114 and a molybdenum/tungsten (II) hydrotris(pyrazolyl)borate system that 
binds to a, P-unsaturated aldehydes in r\ and r| 2 -modes through the carbonyl moiety only 
is also known. 115 The synthesis and crystal structure of a thermally stable, base-free, 
monomeric molybdenum (IV) cz's-ri 4 -methyl vinyl ketone complex, [(Mo(NPh)-r| 4 - 
(0=C(Me)CH=CH 2 )(o-(Me 3 SiN) 2 C6H4)] (78), is discussed below. 

Ph 



N 
111*0- 
Me 3 Si- N Mo 

Me 3 Sif/ 



Ph 

Me 



a 

Me 



Me 3 Si 



b ^ 





Me a 
3 Meg 

77b 



Me 3 Si^ N „Mo 

Me 3 Sif-/ ^ Me * 
Me, 



77d 




Figure 4-13. Possible methyl group exchange pathway 

Pentane solutions of 37 or 38 reacted with 1 .0 equiv of MVK affording 78 in high 
yield. Complex 78 was isolated by removal of the pentane solvent under reduced 
pressure and could be recrystallized from a concentrated toluene/pentane solution at 
-30°C. 

An X-ray crystallographic study on a single crystal of 78 was carried out. The 
thermal ellipsoid plot of 78 is shown in Figure 4-14 and is accompanied by selected bond 
lengths and angles. The crystal data and details of the structure refinement are 
summarized in Table 4-1. The Mo-N(l) bond length of 1.7607(14) A is consistent with a 



87 

metal-nitrogen triple bond. The Mo-O(l) bond is slightly longer (ca. 0.1 A) than a Mo-0 
single bond, while the remaining Mo-MVK carbon contacts (Mo-C(19) 2.3666(17) A, 
Mo-C(20) 2.3702(18) A, and Mo-C(21) 2.2149(19) A) are similar in length to analogous 
Mo-carbon bond lengths in butadiene complex 73, supporting considerable 
r\ -l-oxa-l,3-diene character in 78. The formal oxidation state at the metal center is thus 
best described as Mo(IV), and the 'H NMR spectrum of 78 is consistent with the 
solid-state structure. 




C22 



Figure 4-14. Thermal ellipsoid plot of 78 (50% probability thermal ellipsoids). Selected 
bond lengths (A), angles (°), and torsion angles (°): Mo-O(l) 2.0492(1 1) 
Mo-C(19) 2.3666(17), Mo-C(20) 2.3702(18), Mo-C(21) 2.2149(19) 
0(1)-C(19) 1.324(2), C(19)-C(20) 1.390(3), C(21)-C(20) 1.414(3), ' 
Mo-N(l) 1.7607(14), Mo-N(3) 2.0107(14), Mo-N(2) 2.0452(14), 
O(l)-C(19)-C(20) 118.02(17), C(19)-C(20)-C(21) 123.09(19), ' 
0(1)-C(19)-C(22) 1 19.38(18), C(20)-C(19)-C(22) 123.27(19), 
C(l)-N(l)-Mo 143.40(12), O(l)-C(19)-C(20)-C(21) 4.3(3). 



88 

Summary 

The synthesis and reactivity of molybdenum butadiene complex 73 has been 
discussed. Structurally this complex is similar to other molybdenum butadiene 
complexes. Reactivity wise, this complex shows parallels with earlier metals, forming 
transient r) 3 -allyl metallacycles when treated with 2-butyne and metallacyclic 77 when 
treated with acetone. While this reactivity places 73 in a group with earlier metal 
butadiene complexes, the C-C bond-forming reaction that occurs during the formation of 
74 is more akin to the reactivity of later metals. Thus, complex 73 occupies a unique 
position among other known transition metal butadiene complexes. 
Early Transition Metal Alkyne Complexes 

The study of early transition metal complexes of alkynes has been a mainstay of 
organometallic chemistry for many years. 116 In general, alkyne complexes are generated 
by ligand displacement reactions or by reduction of a metal halide species in the presence 
of the appropriate alkyne. An important development in this area was the discovery that 
n -alkyne ligands have the ability to act as 2 or 4 electron donor ligands and that l3 C 
NMR spectroscopy could be used to determine the electron donating character of the 
alkyne ligand. 117 

The simple bonding scenario for a transition metal alkyne complex is shown in 
Figure 4-15. Both acetylene 7r-bonding MOs can interact with metal-based orbitals 
through a and n-donation interactions if the metal orbitals are available for bonding. 
Back bonding (n-acceptance) interactions are often common in alkyne complexes, and 
when such contributions to bonding are significant, the metallacyclopropene structure is a 
good description of the net alkyne-metal interaction (Figure 4-15). The 8-interaction 



89 



shown in Figure 4-15 does not contribute much to the overall bonding picture, for it is a 
weak interaction. While an extensive foray into the synthesis and characterization of 
Group 6 alkyne complexes has been made over the years, base-free, high-valent examples 
of such are rare entities in the literature. 



M-) 



metallacyclopropene 



7t 



M 



I 



a-donation (n to 
M-g) 




7T-donation (71 to 



<^ 



71 

OO z 

7r-acceptance (M-d xz 
ton ) 



<\> 



/ 



-X 



7T 




5-acceptance (M-d yz 

* 
ton ) 



Figure 4-15. Bonding in transition metal alkyne complexes 

Synthesis and Characterization of Molybdenum Imido-Diamido Alkyne Complexes 
[(Mo(NPh)-Ti 2 -(RCCR)(0-(Me 3 SiN) 2 C 6 H 4 )] (R = Me (75), Ph (79), SiMe 3 (80)) 

We have recently prepared base-free molybdenum (IV) alkyne complexes of the 
type [(Mo(NPh)-T 1 2 -(RCCR)( -(Me 3 SiN) 2 C 6 H 4 )] (R = Me (75), Ph (79), SiMe 3 (80)) 
(Figure 4-16). These complexes were prepared by treating pentane solutions of 37 or 38 
with the appropriate alkyne, followed by removal of solvent under reduced pressure. The 
alkyne reactants effectively displace the bound olefin of 37 or 38 with complete 



90 

conversion to products within 15 min at 20°C. The ! H NMR spectra of 75, 79, and 80 are 
consistent with monomeric alkyne complexes. The carbons of the alkyne fragment 
coordinated to the metal in 75, 79, and 80 resonate at 181 ppm, 180 ppm, and 205 ppm, 
respectively, in the 13 C NMR spectra, suggesting that the alkyne ligands are acting as 3-4 
electron donor ligands. 117 Compounds 79 and 80 are air sensitive but stable in solution 
and in the solid-state. In contrast, 75 is highly unstable and decomposes in solution at 
-40°C overnight. 

Ph Ph 



/ 



/ 



N N 

Me 3 Sk XT [II Ri w — B Ji> R < 7 R 2 

U J{ PentaneRT Me 3 Si-r-N \ ){ 

H ffi R H H 





R, = Me,R 2 = H(37), 

*.+rmm US?* * 

Figure 4-16. Synthesis of [(Mo(NPh)-n 2 -(RCCR)(o-(Me 3 SiN) 2 C 6 H4)] (R = Me (75), Ph 
(79), SiMe 3 (80)) 

Single crystal X-ray analyses were carried out on single crystals of 75 (Figure 4-17) 
and 80 (Figure 4-18). The crystal data and details of the structure refinement are 
summarized in Table 4-2. Both 75 and 80 adopt square pyramidal geometries where the 
imido groups occupy the apical positions. The metal imido interactions (75: Mo-N(l) 
1 .75 1 1(1 7) A, 80: Mo-N(l) 1 .745(2) A) are consistent with metal-nitrogen triple bonds, 
and the molybdenum amide contacts are consistent with metal-nitrogen single-bond 
interactions. 27 ' 46 ' 51 ' 56 - 91 

The Mo-C(20) and Mo-C(21) bond lengths of 2.056(2) A and 2.052(2) A for 75, 
respectively, are consistent with Mo-C single bonds and the C(20)-C(21) bond length of 



91 

1 .297(3) A is close to the generally accepted value of 1 .34 A for a C-C double bond. 1 18 
Pronounced back bonding is responsible for the lengthening of this alkyne bond, and the 
structure of 75 is thus best described as having a strong contribution from a 
metallacyclopropene structure (Figure 4-17). The alkyne ligand can be considered as a 
dianionic ligand, and the formal oxidation state of the metal would be best described as 
Mo(VI). Similar structural parameters have been reported for high-valent Group 4 
alkyne complexes ' and for isostructural tungsten complexes made in our laboratories. ' ' ' 

Similar arguments apply to the solid-state structure of 80. The C(19)-C(20) bond 
length of 1 .305(4) A in 80 is very close to the C(20)-C(21) length of 1 .297(3) A in 75. 
Complex 80 is thus also best described by the metallacyclopropene structure (Figure 
4-18). The unique bonding interactions in these complexes are currently being studied 
theoretically through DFT analysis and these results will be published in due course. 

Reaction of [(Mo(NPh)-T 1 2 -(PhCCPh)(o-(Me 3 SiN) 2 C 6 H4)] (79) with tert-Butyl 
Isocyanide 

When a sample of 79 was treated with 1.0 equiv of Bu'NC, a new metal-containing 

complex formed, as observed by 'H NMR spectroscopy. This complex was not fully 

characterized, but we have assigned it a preliminary structure (81, Figure 4-19), which is 

consistent with the 'H NMR spectrum (selected 'H NMR for 81 (C 6 D 6 , 18 °C): 5 4.05 (br, 

C=C(Ph)H), 2.58 (d, 14 Hz, metallacycle methylene C-H), 1.73 (d, 14 Hz, metallacycle 

methylene C-H)). Complex 81 converted to 82 over the course of several weeks (Figure 

4-19). Complex 82 was characterized by NMR spectroscopy, and the structural 

assignment is shown in Figure 4-20. The chemical shift of the imino-acyl carbon is 

atypical and approximately 60 ppm upfield from where expected. This shift could 

possibly be explained by the unusual metallacyclic nature of 82. 



92 






C13 




Figure 4-17. Thermal ellipsoid plot of 75 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.7511(17), Mo-N(2) 2.0314(16), 
Mo-N(3) 2.0014(16), Mo-C(20) 2.056(2), Mo-C(21) 2.052(2), C(20)-C(21) 
1.297(3), C(20)-C(21)-C(22) 140.2(2), C(21)-C(20)-C(19) 137.3(2), 
C(l)-N(l)-Mo 159.22(15), N(3)-Mo-N(2) 84.41(7), N(3)-Mo-C(20) 
100.89(7), N(2)-Mo-C(21) 93.90(7). 



93 




C23 



C26 



Figure 4-18. Thermal ellipsoid plot of 80 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.745(2), Mo-N(2) 2.022(2) 
Mo-N(3) 2.009(2), Mo-C(20) 2.079(3), Mo-C(19) 2.078(3), C(19)-C(20) 
1.305(4), C(20)-C(19)-Si(3) 147.2(2), C(19)-C(20)-Si(4) 148.6(2), 
C(l)-N(l)-Mo 164.6(2), N(2)-Mo-N(3) 84.51(9), N(2)-Mo-C(20) 
98.96(10), N(3)-Mo-C(19) 96.62(10). Complex 80 crystallized with two 
molecules in the asymmetric unit and only one is shown here. 















94 



Table 4-2. X-ray data a for crystal structures 75 and 80 



75 



80 



Chemical formula 
Formula weight 
Crystal system 
Space group 
H(Mo-Ka) (mm" 1 ) 
a (A) 

ft (A) 

c(A) 

a 

Pi°) 

Y 

V, (A 3 ) 

Z 

Total reflections 

Uniq. reflections 

tf(int) 

Ri [/>2a(/)data] b 

wtf 2 (all data) c 

Larg. diff. peak, hole 



C 22 H33N 3 MoSi2 
491.63 
Triclinic 
P\ 
0.632 
10.7483(5) 
10.8358(6) 
12.3049(6) 
77.788(1) 
64.377(1) 
77.812(1) 
1251.2(1) 
2 
27.50 
8547 
5542 
0.0357 
0.0275 
0.0742 
0.377, -0.366 



C 2 6H45N 3 MoSi4 
607.95 
Triclinic 
P\ 
0.555 
12.9348(7) 
14.2840(7) 
19.643(1) 
79.211(1) 
70.947(1) 
89.995(1) 
3362.6(3) 
4 
27.50 
22991 
15040 
0.0505 
0.0426 
0.1233 
0.499, -0.579 



Obtained with monochromatic Mo Koc radiation (k = 0.71073 A) at 173 K b ./?, = sIIfJ 
Fjl/zlFj. c w* 2 = {I[ W (F 2 - F c 2 ?/Z[w(F 2 ) 2 ]} m . 















95 



Ph 



Ph 



/ \ v 

N N CX 

I] 1 P u 1.0 CN-Bu 1 \y 

N / \!fi Me 3»i^N / 

Me 3 Si7— N X , PentaneRT / n 

H Ph h \ 



Me 3 Si 



%/ 



79 



Me 3 Si^ N -Mo 






Ph 

H 
Ph 



Figure 4-19. Reaction of [(Mo(NPh)-r) 2 -(PhCCPh)(o-(Me 3 SiN)2C 6 H4)] (79) with Bu'NC: 
formation of 82 



0.61,28.9 












Ph 



58.6 



0.59 Me 3 Sk 

N Mo^ 




0.59, 0.94 24.7 



2.05,2.27 



82 

Figure 4-20. Proposed structure of 82 showing selected carbon (underlined) and proton 
chemical shifts, assigned by NMR spectroscopy 



96 

We have no concrete evidence for a possible mechanism but propose that C-H 
activation of the SiMe3 ligand is an important step in the reaction and could be 
responsible for generating a species such as 79a (Figure 4-21). Intermediate 79a could 
come about by insertion of diphenyl acetylene into a metal hydride produced by C-H 
activation of the SiMe 3 methyl group. A y-abstraction process could also be responsible 
for the formation of 79a. The exact role of Bu'NC in this reaction is unknown, but it 
most likely promotes C-H activation. Related Lewis base promoted a, p\ and 
Y-abstraction processes have been observed with the analogous tungsten system, as 
discussed in Chapter 1 , 42-45 Reductive elimination from 79a would result in the 
formation of 81, and insertion of Bu'NC into the metallacyclopropane function of 81 can 
give 82. Full characterization of 81 and 82 will contribute to the formulation of a more 
complete mechanism. 
Summary 

To our knowledge, reports of other metal imido-diamido alkyne complexes are 
nonexistent. The disubstituted alkyne complexes 75, 79, and 80 have interesting bonding 
properties, a result of the Ti-loaded system. A detailed DFT study aimed at understanding 
the bonding in these complexes is underway in our group, as are reactions involving 
monosubstituted and unsymmetrically disubstituted alkyne reactants. Initial reactivity 
studies have shown that complex 79 is a reactive species. We hope to further develop 
this chemistry in the future. 



97 



Ph 



N 






1.0 



Me 3 Si-^ N -Mo ; ^ 1 CNH 




Me.Si 



79 



Bu l 

I 

N 

"J Ph Ph 

\ 
Ph— N^< 



Mo- 



H 






Me 3 Si^ N '/ /^ H 
N— Si 

/ \ 



79a 








Me.Si 



82 




Figure 4-21 . Proposed mechanism for the formation of 82 



CHAPTER 5 
REACTIVITY OF MOLYBDENUM OLEFIN AND ARENE COMPLEXES WITH 

UNSATURATED SUBSTRATES 

Reaction of Molybdenum Olefin Complexes with Imines: The Synthesis of 
Molybdenum Imido-Diamido r| 2 -Imine Complexes 

Current synthetic methodologies affording r) 2 -imine complexes 
(azametallacyclopropanes) consist of C-H activation from methylmetallocene 
amides, l21 rearrangements of iminoacyl complexes, 5122 reaction between Cp* 2 ZrH 2 
and ArNC, reduction of a low-valent complex with phosphaazaallene, 124 and various in 
situ methods of n, -imine complex formation. l2S Isolation and characterization of 
r\ -imine complexes generated by direct reaction of imines with metal reductants has, for 
the most part, been unsuccessful, and although two recent reports detail the isolation and 
characterization of ytterbium 125 and tantalum 126 r) 2 -imine complexes via such direct 
methods, examples with other early metals are, to our knowledge, nonexistent. We report 
herein structural characterization and direct synthesis of molybdenum (IV) r) 2 -imine 
complexes obtained by treatment of the molybdenum (IV) olefin complex, 38, with 
appropriate aldimines and ketimines. Furthermore, aldimine reductive coupling products 
are isolated for less sterically demanding aldimines. 

Reaction of the aryl amine derived aldimine PhN=C(H)Ar (Ar - C 6 H 4 -/?-OMe) and 
ketimine PhN=C(Me)Ph with 38 afforded the n. 2 -imine complexes [Mo(NPh)-r) 2 - 
PhN=C(H)Ar(o-(Me 3 SiN) 2 C6H4)] (Ar = C 6 H 4 -/?-OMe) (83) and [Mo(NPh)-r| 2 - 
PhN=C(Me)Ph(o-(Me 3 SiN)2C 6 H 4 )] (84) as green crystals in 70% isolated yield (Figure 






98 



99 



5-1). The molecular structure of 84 was determined by an X-ray crystallographic study, 
and selected bond distances and angles for 84 are shown in Figure 5-2. The crystal data 
and details of the structure refinement are summarized in Table 5-1. 

R x /Ph 

Ph /C=N ph 

/ Ar / 

N N 

Me 3 Sk. T Ml Me ,Sk III / Ph 

w e- N— Mo — // 3 N— Mo N + 

Me3Sir-- N II Me 3 Si-rHsr NJ r w . 

fa »*iL h r Me Me 



R = H,Ar = C 6 H 4 -/?-OMe(83) 

R - Me, Ar = Ph (84) 

Figure 5- 1 . Synthesis of n 2 -imine complexes 83 and 84 

Complex 84 adopts a five-coordinate square pyramidal geometry with the imido 
ligand occupying the apical position. The short imido Mo(l)-N(3) bond length of 
1 . 73 6(2) A is typical of a Mo-N triple-bond interaction. ' ' 27 ' 47 ' 5 ' The Mo( 1 )-N-(4), 
Mo(l)-N(2), and Mo(l)-N(l) bond lengths of 1.944(2) A, 2.009(2) A, and 1.996(2) A, 
respectively, are all consistent with Mo-N single bonds. 27 ' 46 ' 51 ' 56 ' 91 The C(31 Reentered 
bond angles of the imine fragment are all less than 120° and support a considerable 
amount of azametallacyclopropane character in 84, as does the N(4)-C(31) bond length of 
1 .414(3) A. 12 The imine fragment C-N bond length for 84 is similar to the related bond 
length of 1.421(7) A for the azatitanacyclopropane complex [Ti(OAr)2(Bu l NCCH2Ph) 
(Py-p-Ph)] (Ar = 2,6-di-^/-butylphenyl), 122(b) but much longer than that of the zirconium 
azametallacyclopropane complex [Cp 2 Zr(PhC(H)NPh)(thf)] (1.375(7) A). 120(b) 




























100 



Table 5-1. X-ray data 3 for crystal structures 84, 85, and 88 





84 


85 


88 


Chemical formula 


C32H 4 oN 4 MoSi2 


C 3 8H53N 5 Mo0 2 Si2 


C 2 5H4,N 3 MoOSi2 


Formula weight 


632.80 


763.97 


551.73 


Crystal system 


Monoclinic 


Monoclinic 


Triclinic 


Space group 


P2,/c 


P2i 


Pi 


u(Mo-£a) (mm" 1 ) 


0.509 


0.440 


0.569 


a (A) 


9.8565(5) 


9.9600(5) 


9.8841(7) 


b{k) 


18.8443(8) 


19.0705(9) 


11.2646(7) 


c(A) 


17.9089(8) 


10.2628(5) 


14.766(1) 


a 


- 


- 


101.370(1) 


fin 


104.6520(10) 


97.351(1) 


96.272(1) 


y 


■ 


- 


115.921(1) 


Vc (A 3 ) 


3218.2(3) 


1933.2(2) 


1413.6(2) A 


z 


4 


2 


2 


'-'max 


26.50 


27.49 


27.50 


Total reflections 


6495 


13886 


9474 


Uniq. reflections 


6495 


8413 


6236 


R(int) 


na 


0.0437 


0.0620 


R\ [/>2a(/)data] b 


0.0431 


0.0420 


0.0380 


w/? 2 (all data) c 


0.0831 


0.0843 


0.1090 


Larg. diff. peak, hole 


0.348, -0.404 


0.345,-0.413 


0.481,-0.789 



"Obtained with monochromatic Mo Ka radiation (k = 0.71073 A) at 173 K b /?i = 2JF I 
Fj/dFj. c w/? 2 = {SWF 2 - F C 2 ) 2 /ZWF 2 ) 2 ]} 1/2 . 



101 



C(16) 




C(22) 



C(23) 



CI28) 

Figure 5-2. Thermal ellipsoid plot of 84 (40% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo(l)-N(l) 1.996(2), Mo(l)-N(2) 
2.009(2), Mo(l)-N(3) 1.736(2), Mo(l)-N(4) 1.944(2), Mo(l)-C(31) 
2.200(3), N(3)-C(13) 1.409(3), N(4)-C(31) 1.414(3), C(31)-C(32) 1.518(4), 
Mo(l)-N(3)-C(13) 163.0(2), N(2)-Mo(l)-N(l) 85.28(9), N(4)-Mo(l)-C(31) 
39.29(9), N(2)-Mo(l)-N(4) 100.09(9), N(l)-Mo(l)-C(31) 99.20(9), 
N(4)-C(3 1 )-Mo( 1 ) 60.55(1 3), N(4)-C(3 1 )-C(32) 1 1 6.9(2), 
N(4)-C(31)-C(25) 116.6(2), C(32)-C(31)-C(25) 116.2(2). 

The 'H NMR spectra of 83 and 84 are highly fluxional at 20°C. We have carried 

out variable temperature 'H NMR studies on 84. At 20°C considerable broadening of 

resonances in the aromatic region are observed in the 'H NMR spectrum. These 

resonances become well defined, 1.0 proton peaks at -65°C. We propose that the 

fluxionality observed is a result of the hindered rotation of the phenyl ring attached to the 

imine carbon. This hindered rotation can produce inequivalent or^o-phenyl protons that 









102 

will appear as doublets integrating for 1.0 H in the aromatic region of the *H NMR 
spectrum. We observe one of these doublets at 5.18 ppm (V c .h = 8.0 Hz) in the *H NMR 
spectrum. This proton is shifted significantly upfield due to the ring current of the nearby 
(o-(Me3SiN) 2 C 6 H4) ligand. We have observed similar upfield shifted resonances for the 
aromatic protons of the styrene fragment for a related styrene complex. 46 Furthermore, 
analysis of the space-filling model of 84 shows a sterically congested environment 
around the phenyl group in question, supporting our claims of hindered phenyl group 
rotation. Two resonances for the SiMe 3 groups are observed from -65°C to 70°C by 'H 
NMR spectroscopy, consistent with our interpretation of the dynamic properties of 84 in 
solution. The imine methyl resonance is observed as a broad signal at 2.34 ppm in the 'H 
NMR spectrum of 84 at 20°C. This broad methyl resonance sharpens considerably at low 
(-65°C) and high (70°C) temperatures, which is also consistent with our interpretation. 
The same dynamics are observed for 83 in solution, as observed by variable temperature 
H NMR spectroscopy. 

Complete conversion of 83 to the corresponding organic amine (PhNHCH 2 Ar) was 
observed by 'H NMR spectroscopy upon exposure of a solution of 83 to an atmosphere 
of 15 psi. H 2 gas at 20°C over a 1 week period. Unfortunately, no catalytic activity was 
observed upon treatment of 83 with excess imine under low pressures (ca. 15 psi.) of H 2 
gas. Complexes 83 and 84 did not react with acetone, 2-butyne, or diphenyl acetylene as 
determined by 'H NMR spectroscopy. 

The reaction of aldimines, derived from less hindered primary amines, with 38 
afforded molybdenum (VI) imido (bis)diamido complexes 85 and 86 from reductive 
imine coupling (Figure 5-3). Complexes 85 and 86 were isolated in 75% yield from cold 



103 

pentane/toluene solutions. Interestingly, only the rac-coupled form of the metal 
complexes were isolated, as ascertained from [ H and 13 C NMR spectroscopy. 

Ph Ph 

J f 

N N 

Me 3 Sk_ T HI Pentane M „. ||| R 

" f 4. X- "if ' <$$ " " 

38 

R - CH 2 CH 3 , Ar = C 6 H 4 -/?-OMe (85) 

R = CH 2 Ph, Ar = C 6 H 4 -/?-OMe (86) 
Figure 5-3. Reductive coupling of imines 

An X-ray crystallographic study was performed on a single crystal of 85. The 
molecular structure and selected bond lengths and angles of 85 are shown in Figure 5-4. 
The crystal data and details of the structure refinement are summarized in Table 5-1 . The 
overall geometry around molybdenum is extremely distorted and is not best described as 
trigonal bipyramidal or square pyramidal. Distorted five-coordinate complexes such as 
this one are best described using a method introduced by Reedijk that relies on 
calculating a x-value for the complex in question. 128 The x-value can then be used to 
assign the structure a position between the two extremes in geometry, trigonal 
bipyramidal and square planar. A x-value of 0.0 corresponds to a pure square pyramidal 
structure, while a x-value of 1 .0 corresponds to a true trigonal bipyramid. The x-value for 
85 is 0.47. Thus, the structure of 85 is best described as occupying a position halfway 
between trigonal bipyramidal and square planar. The Mo-N(l ) bond length of 1 .754(3) A 
is consistent with a molybdenum nitrogen triple-bond interaction. 1 '' 27 ' 47 - 51 The Mo-N(2), 
N(3), N(4) and N(5) amide bond lengths of 2.008(3) A, 2.065(3) A, 1.996(3) A, and 
2.021(3) A, respectively, are within the range expected for Mo-N single bonds. 2746 5156 - 91 



104 

The hydrogen atoms located on the backbone of the newly formed diamide ligand are 
related by an H-C(19)-C(29)-H torsion angle of 90°. A consequence of this torsion angle 
is the lack of coupling between these inequivalent (3-protons in the solution ! H NMR 
spectrum at 300 MHz. Another interesting feature in the "H NMR spectrum of 85 is the 
four doublet of quartets observed for the four inequivalent methylene protons at 3.26 
ppm, 3.81ppm, 4.01 ppm, and 4.19 ppm. 




•-*% 



Figure 5-4. Thermal ellipsoid plot of 85 (50% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.754(3), Mo-N(2) 2.008(3), 
Mo-N(3) 2.065(3), Mo-N(4) 1.996(3), Mo-N(5) 2.021(3), N(4)-C(19) 
1.476(5), N(5)-C(29) 1.468(5), C(19)-C(29) 1.514(5), Mo-N(l)-C(l) 
166.6(3), N(2)-Mo-N(3) 80.65(14), N(3)-Mo-N(5) 85.75(12), 
N(5)-Mo-N(4) 77.46(12), N(2)-Mo-N(4) 102.59(12). 

The H NMR spectrum of complex 86 is complicated due to overlapping 
resonances. The resonances in the ! H NMR spectrum have been assigned with the aid of 



105 

GHMBC and GHMQC spectra. The NMR properties of 86 are similar to those of 85. 
The four inequivalent benzyl methylene protons appear as doublets with 1 5 Hz geminal 
coupling constants and resonate at 4.51 ppm, 4.64 ppm, 5.45 ppm, and 5.51 ppm in the 
H NMR spectrum. The (3-protons of the newly formed diamide resonante at 4.75 ppm 
and 4.50 ppm, and although these protons are inequivalent, no coupling is observed at 
500 MHz in the 'H NMR spectrum. 

In summary, we have demonstrated that chelate-supported Mo(IV) r) 2 -imine 
complexes are easily prepared via displacement of olefin from 38 upon reaction with 
N-aryl imines. In contrast, imine reductive coupling products are observed for sterically 
less demanding imines. 

Reaction of Molybdenum Olefin Complexes with Acetone and Aldehydes: The 
Synthesis of Oxametallacyclopentanes 

Transition metal-mediated reductive coupling of a pair of unsaturated molecules 
can result in the formation of a metallacyclopentane with concomitant formation of a 
carbon-carbon bond (Figure 5-5). Reactions of this type performed with in situ-generated 
early metal metallocenes (e.g., Cp 2 Ti, Cp 2 Zr, and Cp 2 Hf) and related reduced metal 
species have been extensively explored. 129 Various protocols have been developed for 
selective metallacycle cleavage, giving rise to an organic product. Although a new 
organic product is formed, a byproduct metallic species in the wrong oxidation state to 
effect further reductive coupling is generated during these proteolytic or oxidative 
cleavage processes. Recent developments in this field that circumvent this problem 
involve the titanium-catalyzed cyclization of enones mediated by silanes 130 and 
titanium-based applications toward the synthesis of y-butyrolactones. 131 



106 



Treatment of the molybdenum olefin complexes discussed above with ketones and 
aldehydes results in coupling of the metal-olefin fragment and the carbonyl moiety, 
giving rise to oxametallacyclopentanes (Figure 5-6). The synthesis and characterization 
of some representative metallacycles are discussed here. 



. Reductive 

T w. ^B Coupling .A-B 

L n M: -* Ln M x _ I 



Oxidative or 
Proteolytic Workup 



Organic Products 

+ 
.X 



L n M 



X 



D : 



:C 



D' 



Catalytic Conditions Organic Products 

+ 
L n M: 

(Required for Catalysis) 
Figure 5-5 . General scheme for the reductive coupling of organic molecules 



Ph 



Ph 



N 



N 



Me 3 Si 



R, 



N— rMo^ -r. 



Me 3 Sir-N // 2 




H 



O 

A 



H 



Pentane Me 3 Sk N -Mo-0 

Me 3 Si7-N >— r 



R, = R 2 = Me (38), 
Ri = Ph, R 2 = H(87) 



Ketone = Acetone, 
3-pentanone, 
or cyclohexanone 



R, - R 2 = Me, R = Me (88), 
R, = Ph, R 2 = H, R = Me (89), 
R, = Ph, R 2 = H, R = Et (90), 
R, = Ph, R 2 = H, Ketone - 
cyclohexanone (91) 

Figure 5-6. Synthesis of oxametallacyclopentanes 

Addition of acetone to a solution of 38 in pentane resulted in the exclusive 
formation of [(Mo(NPh)(C(Me) 2 CH 2 C(Me) 2 0)( -(Me 3 SiN) 2 C 6 H4)] (88), as determined 
by H and 13 C NMR spectroscopy. Single crystals of 88 were grown from cold 
toluene/pentane solutions. 















107 

A single crystal X-ray study of 88 reveals the square pyramidal geometry about 
molybdenum (Figure 5-7). The crystal data and details of the structure refinement are 
summarized in Table 5-1 . The regioselectivity of this particular reaction, with respect to 
the formation of the metallacycle, became quite clear upon examination of the thermal 
ellipsoid plot of 88. The methylene carbon (C(20)) has taken up position between both 
tertiary carbons C(19) and C(21). The propensity for molybdenum to form Mo-0 bonds, 
along with the minimization of steric repulsion between the methyl groups of acetone and 
the isobutene ligand in 38 during the reaction, explain the observed regioselectivity. The 
Mo-N(l) distance of 1 .7348(1 8) A is consistent with a metal-nitrogen triple bond and the 
Mo-N(2) and N(3) amide distances of 2.0513(17) A and 2.0072(18) A, respectively, are 
within the range expected for Mo-N single bonds. The Mo-O(l) distance of 1 .9344(15) 
A and the Mo-C(19) distance of 2.209(2) A are both consistent with metal-atom single 
bonds. 132 

When acetone was added to a dark green solution of 87 in pentane, the solution 
turned dark red in color. Concentration of the pentane solution under reduced pressure 
afforded a red solid. Solution 'H NMR spectroscopy of this sample confirmed the 
existence of only one metallacycle-containing product, [(Mo(NPh)(C(H)PhCH 2 C(Me) 2 0) 
(o-(Me3SiN)2C 6 H 4 )] (89). Pure 89 was obtained via recrystallization from pentane at 
-30°C, and the stereochemistry of 89 was elucidated by NMR spectroscopy (GHMQC, 
GHMBC, and NOESY). The proposed structure for 89 is illustrated in Figure 5-8, and 
the proton chemical shifts have been assigned. Complex 89 adopts a five-coordinate 
square pyramidal geometry very similar to that of 88. Of interest in 89 is the position of 
the C(H)Ph fragment, which was once part of the coordinated styrene in 87. The carbon 



108 



atom of this fragment is bound to the molybdenum center, and the phenyl group occupies 
a syn position in reference to the phenyl imido group. Steric constraints during the 
reaction of 87 with acetone most likely dictate the ultimate configuration of this 
metallacycle. As 89 is the only diastereomer formed, this reaction does proceed with 
high diastereomeric excess. 







C25 



Figure 5-7. Thermal ellipsoid plot of 88 (30% probability thermal ellipsoids). Selected 
bond lengths (A) and angles (°): Mo-N(l) 1.7348(18), Mo-O(l) 1.9344(15) 
Mo-N(3) 2.0072(18), Mo-N(2) 2.0513(17), Mo-C(19) 2.209(2), 0(1)-C(21) 
1.432(3), C(19)-C(20) 1.541(3), C(20)-C(21) 1.534(3), Mo-C(19)-C(20) 
103.80(14), Mo-0(l)-C(21) 122.89(13), C(22)-C(19)-C(23) 109.7(2), 
C(24)-C(21)-C(25) 109.7(2), C(19)-C(20)-C(21) 111.91(18). 



109 



6.85 



0.38 Me.Si 

\ 



-0.06 Me 3 Si x N- 7 Mo v 

tni 





6.91 t 7.30 
7.17 

Figure 5-8. Structural assignment of 89 including assigned proton chemical shifts 

Compounds 90 and 91 were not fully characterized by NMR spectroscopy in terms 
of stereochemistry. Their structures are, however, most likely similar to 89, and the 
reactions proceed with high diastereomeric selectivity as determined by "H NMR 
spectroscopy. 

The reaction of 38 with p-anisaldehyde results in the formation of only one 
metallacyclic product, 92. Although not yet fully characterized via NMR techniques, 
only one diastereomer is formed in this particular reaction. 

Reaction of 37 or 87 with aldehydes gives rise to oxametallacyclopentanes. The 'H 
NMR spectra of these reaction mixtures are complex due to the formation of multiple 
diastereomers, indicating the low diastereomeric selectivity of reactions between 
monosubstituted olefin complexes and aldehydes. Because of this low selectivity, these 
reactions were not investigated further. 

Reactivity of Arene Complexes with Acetone 
We have observed differing reactivity between arene complexes 40 and 44 with 
acetone. The benzene complex 40 reacts with acetone affording n 2 -ketone complex 93 



110 

and 94 in a 1 :2 ratio (Figure 5-9). Complex 94 most likely arises as a result of a C-H 
activation pathway. In sharp contrast, reaction of 44 with acetone produces only 93. 

The structures of 93 and 94 have been proposed based on simple *H and 13 C NMR 
data. A resonance at 90.5 ppm in the 13 C NMR spectrum of 93 is observed at 20°C and 
has been assigned to the carbon of an r| 2 -coordinated acetone moiety. The acetone 
methyl group protons of 93 are observed as a broad resonance at 1.01 ppm in the *H 
NMR spectrum, and the SiMe 3 protons resonate at 0.49 ppm in the same spectrum. 
Characteristic resonances in the ! H NMR spectrum of 94 are the septet at 4.57 ppm 
(assigned to H a , 3 J C -h = 6.0 Hz) and the two doublets at 0.76 ppm and 1 .44 ppm (V C . H = 
12.5 Hz), assigned to H b and H c . The methyl groups that couple to H a are observed as 
overlapping doublets at 1 . 10 ppm ( 3 J C -h = 6.0 Hz). Complete characterization of these 
products will be required. 

We can only assume that 40 is in equilibrium with a metallated hydride species 
(95), which reacts with acetone giving 94 (Figure 5-10). In contrast, 44 is not in 
equilibrium with a metallated species due to steric constraints. 



















Ill 



Ph 



N 



Me 3 Si^ N - 
Me 3 Si7— N 



Mo. 




40 



Me 3 Si^ N - t 
Me 3 Si^N 




44 



Ph 



O 




N 
III 
Me 3 Si^ N ^o. 

Me 3 Si r -N 



O 



C 6 D 6 , 20 °C 




93 




O 



Ph 



N 



C 6 D 6 ,20°C Me 3 Si- N ^o, 

Me 3 Si^N u 




93 



Ph FL 




N 

III 

Me 3 Si^ N Mo 



-O' 





+ 





Figure 5-9. Reaction of arene complexes with acetone 

Ph 



N 
III 
Me 3 Si^Mo. 



Me 3 SiT-N 





Ph 



N 



.H 



C 6 D 6 , 20 °C 



Me 3 S^ N Mo^ 




\ 




40 95 

Figure 5-10. Possible equilibrium between an arene complex and a metallated species 












CHAPTER 6 
EXPERIMENTAL DATA 

General Methods 

All reactions were conducted under a dry argon atmosphere using standard Schlenk 
techniques, and all compounds were handled in a nitrogen-filled dry box. All solvents 
were distilled under nitrogen from sodium or sodium benzophenone ketyl or passed over 
activated alumina, stored over molecular sieves, and degassed prior to use. 

NMR spectra were obtained on a Varian Gemini 300, VXR 300, or Mercury 300 
instrument with C 6 D 6 , C 7 D 8 , or CDC1 3 as solvents, as noted, and referenced to residual 
solvent peaks. A Varian Inova 500 equipped with an indirect detection probe was used as 
indicated for the GHMQC, 133 GHMBC, 134 and NOESY 135 experiments. The full X-ray 
diffraction data for all compounds can be obtained by contacting Dr. Khalil A. Abboud at 
352-392-5948. 

Density functional theory (DFT) calculations were performed using the Gaussian 
98 W program package. 53 Structures were optimized using Becke's hybrid 
three-parameter functional (B3LYP). The Los Alamos effective core potential (ECP) 
plus a standard double-zeta basis set (LANL2DZ) was used to describe the molybdenum 
and silicon atoms. 136 Dunning/Huzinaga full double zeta basis set (D95) was used to 
describe all other atoms. 136 Using the same level of theory and basis set, vibrational 
frequency calculations and full population analyses were performed on the stationary 
points to negate transition structures, to determine thermal/zero-point energies, and to 
analyze (MOs). ' Thermochemical information at temperature T = 298. 1 5 K and P = 

112 



113 

1 .00 atm was obtained using frequencies without scaling. Single-point energies for each 
structure were calculated using the B3LYP level of theory and the LANL2DZ basis set, 
with restricted wave functions again being used for the closed-shell species. 

Complex 19 was synthesized according to published procedure, and crystals were 
grown from a cold toluene/pentane solution. 27 The synthesis, details of X-ray structure 
data, and characterization of 52 and 53 have been reported elsewhere and are included in 
this Chapter to reflect recent updates. 66 ' 91 Ligand 58 was synthesized from the 
imidazolium salt according to standard procedure. 67 The imidazolium salt was purchased 
from Strem Chemicals. 

Synthesis and Characterization 

[Mo(NPh)(Py) 2 (0-(Me 3 SiN) 2 C 6 H 4 )](47) 

A representative synthesis of 47: To a green pentane solution of freshly generated 
38 (1.07 g, 2.18 mmol) was added excess pyridine (0.51 g, 6.54 mmol). Upon addition of 
pyridine, 47 precipitated from solution and was isolated by filtration in 90% yield as a 
purple powder. 'H NMR (C 6 D 6 , 18°C): 5 0.38 (SiM> 3 , 18 H), 5.93 (t oft, 7.5 Hz, 1.0 Hz, 
pyridine para protons), 6.19 (t, br, 6.0 Hz, pyridine meta protons, 4 H), 6.7 (very br, 
pyridine ortho protons, 2 H), 6.90 (t oft, 7.0 Hz, 1.0 Hz, para proton), 7.07 (t oft, 8 Hz, 
1 Hz, meta protons), 7.12 (m, o-(Me 3 SiN) 2 C 6 // 4 protons, 2 H), 7.24 (d, ortho protons), 
7.51 (m, o-(Me 3 SiN) 2 C6//4 protons, 2 H), 8.56 (very br, pyridine ortho protons, 2 H). 'H 
NMR (C 7 D 8 , -55°C): 5 0.41 (SiM> 3 , 18 H), 5.88 (t, 7 Hz, pyridine para protons, 2 H), 
6.12 (t, 7 Hz, pyridine meta protons, 2 H), 6.18 (t, 7 Hz, pyridine meta protons, 2 H), 6.52 
(d, 5.5 Hz, pyridine ortho protons, 2 H), 6.9-7.2 (ov, m, 7 H), 7.54 (m, o-(Me 3 SiN) 2 C 6 // 4 
protons, 2 H), 8.64 (d, 5.5 Hz, pyridine ortho protons, 2 H). 13 C{'H} NMR (C 6 D 6 , 18°C): 



114 

S 5.0, 117.1, 117.6, 122.7, 123.2, 124.0, 129.0, 130.0, 143.5 (br), 152.7, 160.4. Anal. 
Calcd for C28H37 N 5 MoSi 2 : C, 56.45; H, 6.26; N, 1 1.76. Found: C, 56.32; H, 6.34; N, 
11.53. 

[Mo(NPh)^a/i5(Py) 2 (CO)(o-(Me 3 SiN) 2 C6H4)](48) 

To a degassed flask containing a toluene solution of 47 (0.200 g, 0.336 mmol) was 
added carbon monoxide gas (ca, 15 psi). Concentration of this solution under reduced 
pressure afforded 48 as a red solid in 89% yield. *H NMR (C 6 D 6 , 20°C): 8 0.32 (SiM? 3 , 9 
H), 0.37 (SiM? 3 , 9 H), 6.30 (t, 6.5 Hz, pyridine meta protons, 4 H), 6.54 (t, 7.5 Hz, 2 H), 
6.7-7.2 (ov, m, 7 H), 7.40 (d, 7.0 Hz, phenyl imido ortho protons, 2 H), 8.69 (d, 5.0 Hz, 
pyridine ortho protons, 4 H). 13 C{'H} NMR (C 6 D 5 Br, 18°C): 3.4, 3.7, 1 14.4, 1 17.4, 
117.8, 119.5, 123.3, 124.6, 128.4, 136.7, 149.6, 151.2, 152.8, 156.0, 268.7, one resonance 
obscured by solvent. IR: 1913 cm" 1 . 
Imido-Bridged, Bimetallic 49 

A toluene solution of 47 (0.200 g, 0.336 mmol) was heated to 90°C in a sealed 
ampule for 2 h. Concentration of the reaction mixture under reduced pressure afforded 
49 as a black solid in 92% yield. ! H NMR (C 6 D 6 , 18°C): 5 0.00 (SiM? 3 , 9 H), 0.1 1 
(SiM? 3 , 9 H), 0.23 (SiM? 3 , 9 H), 0.31 (SiM? 3 , 9 H), 6.2 (br, 1 H), 6.45 (t, 7.0 Hz, 1 H), 
6.6-7.0 (ov, m, 9 H), 7.33 (t, 7.5 Hz, 2 H), 7.6 (m, 2 H), 7.94 (d, 7.5 Hz, 2 H). I3 C{'H} 
NMR(CDC1 3 , 18T): 5 2.2, 3.0, 3.1, 3.7, 116.8, 118.5, 118.6, 118.7, 118.8, 121.2, 123.0, 
123.3, 123.6, 124.4, 126.0, 126.6, 127.4, 128.2, 132.8, 137.5, 150.0, 150.9, 151.8, 152.7, 
160.1, 161.0, 169.9. 



115 

[Mo(NPh)(P(OMe)3)3(o-(Me3SiN) 2 C 6 H 4 )](50) 

A representative synthesis of 47: To a green pentane solution of freshly generated 
38 (0.85 g, 1.72 mmol) was added excess P(OMe 3 ) 3 (0.81 mL, 6.88 mmol). Upon 
addition of P(OMe 3 )3 red-brown 50 precipitated from solution and was isolated by 
filtration in 83% yield. Complex 50 is highly unstable and decomposes rapidly in 
solution and in the solid-state. 'H NMR (C 6 D 6 , 18°C): 6 0.62 (SiM? 3 , 18 H), 3.20 (d, 10 
Hz, P(OM? 3 ) 3 , 27 H), 6.85 (m, phenyl imido para proton), 7.02 (m, 4 H), 7.48 (m, 4 H). 
13 C{'H}NMR(C 6 D 6 , 18°C): 6 5.3, 50.7, 117.8, 119.5, 124.6, 125.6, 128.3, 151.5, 157.8. 
3, P{'H} NMR (C 6 D 6 , 18°C): 136.0 (br), 196.0 (br). 
[Mo(NPh)(P(OMe) 3 )2CO( -(Me3SiN) 2 C 6 H 4 )](51) 

To a degassed flask containing a toluene solution of 50 (0.800 g) was added carbon 
monoxide gas (ca, 15 psi). Concentration of this solution under reduced pressure 
afforded 51 as a red solid in 87% yield. 'H NMR (C 6 D 6 , -50°C): 5 0.66 (SiMe 3 , 9 H), 
0.75 (SiM? 3 , 9 H), 3.03 (P(OM> 3 ) 3 , 18 H), 6.79 (m, 2 H), 7.05 (ov, m, 4 H), 7.44 (d, 8 Hz, 
1 H), 7.54 (d, 8 Hz, 2 H). I3 C{'H} NMR (C 6 D 6 , 18°C): 6 4.8 (br), 51.6, 1 13.7 (br), 1 19.1 
(br), 125.0, 125.8, 129.2, 148.0, 157.3, 250.9. 31 P{'H} NMR (C 7 D 8 , -50°C): § 131.7. IR: 
1959 cm -1 . 

[Mo(NPh)(PMe 3 )3MMe 3 SiN) 2 C 6 H4)](52) 

To a stirring pentane solution of 37 (1 .0 g, 1.83 mmol) was added PMe 3 (0.57 mL, 
5.53 mmol). A purple solid precipitated from solution and was isolated by filtration and 
washed twice with 10 mL of pentane, affording analytically pure 52. Complex 52 is best 
stored at -30°C under an inert atmosphere. X-ray quality crystals were grown in toluene 
at -30°C. 'H NMR (C 7 D 8 , -50°C): 5 0.35 (br, SiMe 3 , 9 H), 0.61 (br, SiMe 3 , 9 H), 0.98 (br, 



116 

PM? 3 , 18 H), 1.25 (d, br, 5 Hz, PM? 3 , 9 H), 6.7-7.1 (ov, m, 8 H), 7.47 (d, 8 Hz, 1 H). 
3, P{'H} NMR: -8.9 (t, 5 Hz), -10.6 (br), -1 1.3 (br). Anal. Calcd for C 2 7H54MoN 3 P3Si2: 
C, 48.71; H, 8.18; N, 6.31. Found: C, 48.54; H, 8.19; N, 8.19. 
Mo(NPh)(PMe 3 ) 3 (0-(Me 3 SiN)(NH)C 6 H 4 )](53) 

A toluene solution of 60 was warmed to 20°C and stirred for 2 h. Solvent was 
removed from the resulting purple solution under reduced pressure, giving crude 53, as a 
solid. Analytically pure 53 was obtained by washing the crude solid with pentane. "H 
NMR (C 7 D g , -70°C): 5 0.75 (ov, ?Me 3 and SiM> 3 , 27 H), 0.98 (d, 7 Hz, ?Me 3 , 9 H), 3.84 
(d, br, 10 Hz, N-H), 6.7-7.2 (ov, m, 8 H), 7.58 (d, 1 H). 13 C{'H} NMR (C 6 D 6 , 20°C): 5.2 
(SiM? 3 ), 19.4(br,PMe 3 ), 113.7, 113.8, 116.3, 117.0, 122.4, 125.6, 129.1, 150.0, 152.2, 
157.9. 31 P{'H} NMR (C 7 D 8 , -70°C): 1 1.4 (t, 9 Hz), 12.0 (d, 9 Hz). Anal. Calcd for 
C 2 4H46MoN 3 P3Si: C, 48.56; H, 7.81; N, 7.08. Found: C, 48.24; H, 7.63; N, 7.37. 
[Mo(NPh)(PMe 3 )2CO(o-(Me 3 SiN) 2 C 6 H 4 )](54) 

To a degassed flask containing a toluene solution of 52 (0.800 g) was added carbon 
monoxide gas (ca, 15 psi). Concentration of this solution under reduced pressure 
afforded 54 as a red solid in 84% yield. ] H NMR (C 6 D 6 , 20°C): 8 0.5 1 (SiMe 3 , 9 H), 0.60 
(SiMe 3 , 9 H), 0.86 (PM? 3 , 18 H), 6.75 (ov, m, 2 H), 6.95 (ov, m, 5 H), 7.10 (d, 8 Hz, 2 
H), 7.34 (d, 8 Hz, 1 H). 13 C{'H} NMR (C 6 D 6 . 18T): 5 5.0, 6.3, 16.1, 1 13.2, 1 18.2, 
118.9, 120.6, 124.2, 124.5, 129.4, 150.2, 152.3, 156.3,260.2. 3, P{'H} NMR (C 6 D 6 , 
18°C):8-15.2. IR: 1925 cm" 1 . 

[Mo(NPh)(PMe 2 Ph) 2 (0-(Me 3 SiN) 2 C 6 H 4 )](55) 

A representative synthesis of 55: To a green pentane solution of freshly generated 
37 (1.53 g, 3.18 mmol) was added excess PMe 2 Ph (1.76 g, 12.72 mmol). Upon addition 









117 

of PMe 2 Ph, green 55 precipitated from solution and was isolated by filtration in 65% 
yield. 'H NMR (C 6 D 6 , -65°C): 5 0.45 (SiM? 3 , 18 H), 0.94 (br, PM? 2 Ph, 12 H), 7.0 (ov, 
m, 17 H), 7.31 (d, 8 Hz, 2 H). Selected ,3 C{'H} NMR (C 7 D 8 , -65°C): 5 4.9, 1 19.0, 121.1, 
124.3, 125.1, 131 (ov resonances), 138 (ov), 149.9, 159.2. 31 P{'H} NMR (C 6 D 6 , 20°C): 5 
28.2. 

[Mo(NPh)(PMe 2 Ph) 2 CO(0-(Me 3 SiN) 2 C 6 H 4 )](56) 

To a degassed flask containing a toluene solution of 55 (0.800 g) was added carbon 
monoxide gas (ca, 15 psi). Concentration of this solution under reduced pressure 
afforded 56 as a red solid in 87% yield. ! H NMR (C 6 D 6 , 20°C): 5 0. 19 (SiM? 3 , 9 H), 0.55 
(SiM? 3 , 9 H), 1.20 (PMe 2 Ph, 12 H), 6.80 (t, 7 Hz, 2 H), 7.0 (ov, m, 15 H), 7.49 (d, 7 Hz, 2 
H). l3 C{'H}NMR(C 6 D 6 ,20 o C):5 5.0,5.7, 16.0 (br), 113.3, 118.7, 119.1, 121.2, 123.4, 
124.5, 128.8, 129.3, 129.5, 131.2, 150.6, 152.1, 156.7, 261.1, one resonance obscured by 
solvent. 3l P{ 1 H}NMR(C 6 D 6 ,20°C):5-7.0. IR: 1921 cm" 1 . 

[Mo(NPh)(DMPE)(o-(Me 3 SiN) 2 C 6 H 4 )-p-(DMPE)Mo(NPh)(DMPE) 

(o-(Me 3 SiN) 2 C 6 H 4 )] (57) 

A representative synthesis of 57: To a green pentane solution of freshly generated 
37 (1.70 g, 3.53 mmol) was added excess DMPE (1.76 g, 10.59mmol). Upon addition of 
DMPE, red 57 precipitated from solution and was isolated by filtration in 93% yield. 'H 
NMR (C 6 D 6 , 20°C): 5 0.46 (SiMe 3 , 36 H), 0.8-1.4 (complex m, DMPE C// 2 and Me, 48 
H), 6.75 (d of d, 6.0 Hz, 3.5 Hz, o-(Me 3 SiN) 2 C 6 // 4 protons, 4 H), 6.84 (t, 7.0 Hz, 2 H), 
7.0 (ov, m, 8 H), 7.13 (d of d, 6.0 Hz, 3.5 Hz, 4 H). 3I P{'H} NMR (C 6 D 6 , 20°C): 5 -42.0, 
50.0. 



118 

[Mo(NPh)IMes(0-(Me 3 SiN) 2 C 6 H 4 )](59) 

To a C 6 D 6 solution of 55 was added 1.0 equiv of imidazol-2-ylidene 58 (IMes). 
Complex 59 formed in high yield as determined by ! H NMR spectroscopy. Single 
crystals of 59 formed in the NMR tube and were isolated for characterization by 'H NMR 
spectroscopy and X-ray crystallography. 'H NMR (C 6 D 6 , 20°C): 5 -38.2 (br, 2 H), -5.01 
(2 H), -2.42 (phenyl imido para proton), 0.86 (mesityl Me protons, 12 H), 3.54 (mesityl 
Me protons, 6 H), 4.48 (br, 2 H), 8.81(2 H), 10.09 (mesityl protons, 4 H), 17.55 (2 H), 
30.00 (SiM? 3 , 18 H). 

[Mo(NPh)(PMe 3 ) 2 (H 2 )(0-(Me 3 SiN) 2 C 6 H 4 )](6O) 

A general synthesis of complex 60: To a resealable NMR tube was added 52 (0.050 
g, 0.075 mmol) in C 7 D 8 at 20°C. The solution was frozen and the headspace of the NMR 
tube was evacuated. The mixture was brought to -10°C and filled with molecular 
hydrogen (ca. 1 atm). The color of the solution changed from purple to green upon H 2 
gas addition. A ratio of 1 :3 60/52 was observed by *H and 3I P NMR spectroscopy at 
-50°C. 'H NMR (C 7 D 8 , -30°C): 5 0.66 (SiMe 3 , 9 H), 0.73 (SiM> 3 , 9 H), 0.79 (virtual 
triplet, separation 3 Hz, PM? 3 ), 3.59 (t, br, %.„ = 28 Hz, H 2 ), 6.7-7.4 (ov, m, 8 H), 7.41 
(d, 1 H). I3 C{'H} NMR (C 7 D 8 , -50°C): 5 4.6 (SiM? 3 , 9 H), 5.8 (SiM? 3 , 9 H), 15.4 (t, 1 1 
Hz, ?Me 3 , 114.1, 116.7, 117.1, 118.4, 123.8, 149.9, 151.8, 155.4, two resonances are 
obscured by solvent. 3I P NMR{ *H} (C 7 D 8 , -30°C): -14.2 (t, br, 28 Hz). 31 P NMR (C 7 D 8 , 
-30°C): 5 -14.2. Note that the procedure for the synthesis of 60D is identical but uses 
H-D gas. 



119 

[Mo(NPh)(PMe 2 Ph)2(H2)(0-(Me3SiN)2C 6 H 4 )](66) 

The synthesis of 66 is identical to that of 60. *H NMR (C 7 D 8 , -30°C): 5 0.39 
(SiM? 3 , 9 H), 0.63 (SiM? 3 , 9 H), 1.02 (PMe 2 Ph, 6 H), 1.09 (PM? 2 Ph, 6 H), 3.46 (br, H 2 ), 
6.78 (ov, m, 2 H), 7.0 (ov, m, 16 H), 7.51 (d, 8 Hz, 1 H). 3, P NMR (C 7 D 8 , -20°C): § -5.0 
(br). 31 P{'H} NMR (C 7 D 8 , -20°C): 5 -5.0. Note that the procedure for the synthesis of 
66D is identical but uses H-D gas. 
[Mo(NPh)(PMe 2 Ph) 2 (o-(Me 3 SiN)(NH)C 6 H4)](67) 

A toluene solution of 66 was warmed to 20°C and stirred for 2 h. Solvent was 
removed from the resulting purple solution under reduced pressure, giving crude 67 as a 
purple solid. *H NMR (C 6 D 6 , 20°C): 5 0.71 (SiMe 3 , 9 H), 1.17 (d, 7.0 Hz, PMe 2 Ph, 12 
H), 5.97 (br, N-/7), 7.0 (ov, m, 18 H), 7.75 (m, 1 H). 
[Mo(NPh)(P(OMe 3 ))3(o-(Me 3 SiN)(NH)C 6 H4)](68) 

To a degassed C 6 D 6 NMR sample of 50 was added molecular hydrogen at 20°C. 
After 1 week, 68 was observed in solution by *H NMR spectroscopy. ! H NMR (C 6 D 6 , 
20°C): 5 0.75 (SiM? 3 , 9 H), 3.23 (d, 1 1 Hz, P(OM0 3 , 27 H), 7.0 (ov, m, 6 H), 7.45 (d, 8.0 
Hz, 2 H), 7.65 (m, 1 H), N-H resonance not observed. 
Synthesis and Characterization of 71 

To a 20°C toluene solution of 53 (0.77 g, 1.15 mmol) was added H 2 SiPh 2 (0.21 mL, 
1.15 mmol). The mixture was stirred overnight and concentrated under reduced pressure. 
The remaining solid was dissolved in a minimum amount of toluene and cooled to -30°C. 
Solid 71 precipitated from solution as a white powder and was isolated by filtration. *H 
NMR (C 6 D 6 , 20°C): 5 0.1 1 (SiM? 3 , 18 H), 6.90 (m, o-(Me 3 SiN) 2 C6^ 4 protons, 2 H), 7.07 



120 

(m, o-(Me 3 SiN) 2 C6#4 protons, 2 H), 7.14 (ov, m, 7 H), 7.85 (ov, m, 3 H). MS (LSIMS): 
432 m/e. FW 71 = 432.78. 

[(Mo(NPh)-Ti 4 -(H 2 C=CHCH=CH 2 )(o-(Me 3 SiN) 2 C6H4)](73) 

For a typical procedure, butadiene gas (ca, 1 5 psi) was added to a degassed flask 
containing a green solution of 37 (0.42 g, 0.88 mmol) in pentane. The mixture was 
stirred for 10 min and concentrated in vacuo affording 73 as a green solid in 90% yield. 
Complex 73 decomposes rapidly in solution at 20°C and is best stored as a solid at -30°C 
under an inert atmosphere. 'H NMR (C 6 D 6 , 20°C): 5 0.33 (SiMe 3 , 18 H), 0.33 (ov, 
butadiene protons, 2 H), 4.20 (m, butadiene protons, 2 H), 5.49 (m, butadiene protons, 2 
H), 6.7-7.0 (ov, m, 7 H), 7.23 (m, aromatic protons, 2 H). 13 C{'H} NMR (C 6 D 6 , 20°C): 5 
4.7,75.0, 114.7, 117.9, 119.0, 119.4, 127.3, 129.3, 152.8, 158.9. 'HNMR(C 7 D 8 , -30T, 
assigned via GHMQC, 500 MHz): 5 0.13 (br, m, terminal butadiene protons, 2 H), 0.33 
(br, SiMe 3 , 18 H), 4.21 (br, m, terminal butadiene protons, 2 H), 5.53 (br, m, internal 
butadiene protons, 2 H), 6.69 (t, 8.0 Hz, para proton), 6.72 (d, 8.0 Hz, ortho protons), 
6.85 (t, 8.0 Hz, meta proton), 6.94 (m, 0-(Me 3 SiN) 2 C 6 // 4 protons, 2 H), 7.25 (m, 



o-(Me 3 SiN) 2 C 6 // 4 protons, 2 H). U C NMR (C 7 D 8 , -30°C, assigned via GHMQC and J< 



CH 



in Hz from the gated decoupled I3 C NMR spectrum): 8 4.2 (q, 122 Hz, SiM? 3 ), 75.0 (br, t, 
158 Hz, butadiene terminal carbon), 1 14.4 (d, 164 Hz, butadiene internal carbon), 1 17.3 
(d, 147 Hz), 1 18.3 (d of d, 157 Hz, 8 Hz), 1 18.7 (d, 162 Hz), 126.9, 128.8, 152.3 (t, 7 
Hz), 158.3 (t, 8 Hz). Due to the thermal instability of 73, elemental analysis was not 
possible. 



121 

[(Mo(NPh)-r) 4 -(2,3-dimethyI-l,3-cyclohexadiene)(o-(Me 3 SiN)2C 6 H4)](74) 

l,2-Dimethyl-l,4-cyclohexadiene (0.145 g, 1.33 mmol) was added to a stirring solution 
of 38 (0.660 g, 1.33 mmol) in pentane. This mixture was stirred for 5 days and then 
concentrated under reduced pressure affording 74 in 76% yield as a green solid. 
Analytically pure 74 can be obtained by washing the solid with pentane at 20°C. *H 
NMR (C 6 D 6 , 20°C, 500 MHz, assigned by GHMQC, GHMBC, and NOESY): 8 0.32 
(SiM? 3 , 18 H), 0.31 (ov, cyclohexadiene methylene protons, 2 H), 1.45 (br, m, 
cyclohexadiene methylene protons, 2 H), 2.05 (Me), 4.59 (br, C=C(H)), 6.68 (ortho 
protons), 6.73 (para proton), 6.83 (m, o-(Me 3 SiN) 2 C 6 // 4 protons, 2 H), 6.93 (meta 
protons), 7.16 (m, o-tMe^iN^C^ protons, 2 H). I3 C NMR (C 6 D 6 , 20°C, 500 MHz, 
assigned by GHMQC and GHMBC): 5 3.6 (SiMe 3 ), 18.4 (CH 2 ), 22.0 (Me), 87.4 
(C=C(H)), 1 17.3 (o-(Me 3 SiN)2C 6 H4), 1 17.6 (ortho carbon), 1 18.2 (o-(Me 3 SiN) 2 C 6 H 4 ), 
126.0 (para carbon), 126.2 (C=C(H)), 128.7 (meta carbon), 153.3 (ipso, 
o-(Me 3 SiN) 2 C 6 H4), 158.7 {ipso, phenyl). Anal. Calcd for C 26 H 39 MoN 3 Si 2 : C, 57.22; H, 
7.20; N, 7.70. Found: C, 57.08; H, 7.26; N, 7.52. 

[(Mo(NPh)-t| 4 -(2,3-dimethyl-l,3-cyclohexadiene)(o-(Me 3 SiN)2C 6 H4)] (in situ) (74) 
An NMR tube containing 73 (136.3 mgs, 27.7 mmol) in C 6 D 6 was charged with 

2-butyne (21.7 uL, 27.7 mmol) at 20°C. The reaction was complete after 30 min. 

Complex 74 was observed in the 'H NMR spectrum of the reaction mixture. 

Intermediates [5v/i-(Mo(NPh)(C(Me)=C(Me)CH 2 CHCHCH 2 )(o-(Me 3 SiN) 2 C 6 H4)] 
(76a) and [fl"//-(Mo(NPh)(C(Me)=C(Me)CH 2 CHCHCH 2 )(o-(Me 3 SiN) 2 C 6 H 4 )] (76b) 

An NMR tube containing 73 (136.3 mgs, 27.7 mmol) in cold C 7 D 8 was charged 
with 2-butyne (21.7 uL, 27.7 mmol). The mixture was frozen with liquid nitrogen after 
1 .0 min and placed in a cooled (-20°C) NMR probe. The intermediates accumulated to 



122 

detectable levels within hours and were identified by NMR spectroscopy. All NMR 
spectra (GHMQC, GHMBC, NOESY, DQCOSY, and TOCSY) were acquired at -20°C 
on an Inova 500. For selected 'H and 13 C NMR data for 76a and 76b see Figure 4-8 and 
Figure 4-9, respectively. 

[(Mo(NPh)(CH 2 CH=CHCH 2 C(Me)20)(o-(Me 3 SiN) 2 C 6 H4)](77) 

An NMR tube containing 73 in C 6 D 6 was charged with 1.0 equiv of acetone. 
Complex 77 formed after 30 min and was characterized by NMR spectroscopy. The 
isolation of complex 77 has not been attempted at this point. For selected NMR data see 
Figure 4-12. 

[(Mo(NPh)-r| 4 -(0=C(Me)CH=CH 2 )(o-(Me3SiN) 2 C 6 H4)](78) 

MVK (0.136 g, 1.945 mmol) was added to a stirring pentane solution of 38 (0.960 
g. 1 .945 mmol). Concentration of the pentane solution under reduced pressure afforded 
solid 78 in 78% yield following crystallization and filtration at -30°C. ! H NMR (C 6 D 6 , 
20°C): 5 0.27 (SiMe 3 , 9 H), 0.52 (SiM? 3 , 9 H), 0.86 (d of d, 7.5 Hz, 4.0 Hz, olefinic 
proton), 2.16 {Me), 4.06 ("t", 8 Hz. olefinic proton), 5.21 (d of d, 9.0 Hz, 1 1.5 Hz, 
olefinic proton), 6.7-7.2 (ov, m, aromatic protons, 9 H). 

[(Mo(NPh)-ri 2 -(MeCCMe)(o-(Me3SiN) 2 C 6 H4)](75) 

To a stirring solution of 38 (0.340 g, 6.880 mmol) in pentane was added 2-butyne 
(0.0373 g, 6.880 mmol). The reaction mixture was stirred for 10 min, and 75 was 
isolated as a red solid upon concentration of the reaction mixture under reduced pressure 
(yield 76%). Complex 75 is not stable in solution at 20°C and also decomposes in 
solution at -30°C overnight. 'H NMR (C 6 D 6 , 20°C): 6 0.45 (SiMe 3 , 18 H), 2.17 
(MeCCMe, 6 H), 6.9-7.2 (ov, m, 9 H). 13 C{'H} NMR (C 7 D 8 , -25°C): 5 1.8, 17.8, 121.9, 






123 

123.3, 123.8, 124.1, 157.8, 181.5. Two resonances are overlapping with solvent. Due to 
the thermal instability of 75, elemental analysis was not possible. 

I(Mo(NPh)-r| 2 -(PhCCPh)(o-(Me3SiN)2C 6 H4)](79) 

To a stirring solution of 38 (0.590 g, 1.195 mmol) in pentane was added diphenyl 
acetylene (0.213 g, 1.195 mmol). The reaction mixture was stirred overnight, and 79 was 
isolated as a red solid upon concentration of the reaction mixture under reduced pressure 
(yield 77%). 'H NMR (C 6 D 6 , 20°C): 5 0.42 (SiM- 3 , 18 H), 6.8-7.4 (ov, m, 19 H). 
13 C{'H} NMR (C 6 D 6 ,20°C): 8 2.1, 122.9, 123.6, 124.6, 125.2, 128.5, 128.7, 129.0, 
134.1, 139.8, 157.8, 182.0. Anal. Calcd for C 3 2H37MoN 3 Si2: C, 62.42; H, 6.06; N, 6.82. 
Found: C, 62.11; H, 6.41; N, 6.46. 

[(Mo(NPh)-r) 2 -(Me 3 SiCCSiMe3)(o-(Me3SiN) 2 C6H 4 )](80) 

To a stirring solution of 38 (0.910 g, 1 .800 mmol) in pentane was added 

bis(trimethylsilyl)acetylene (0.307 g, 1.800 mmol). The reaction mixture was stirred 

overnight, and 80 was isolated as a green solid upon concentration of the reaction mixture 

under reduced pressure (yield 77%). 'H NMR (C 6 D 6 , 20°C): 5 0.22 (SiM? 3 , 18 H), 0.41 

(SiM? 3 , 18 H), 6.08 (t oft, 7.0 Hz, 1.0 Hz, phenyl imido para proton), 6.9 (m, 

o-(Me 3 SiN)2C 6 //4 protons, 2 H), 7.05 (t, 8.0 Hz, phenyl imido meta protons, 2 H), 7.3 

(ov, m, o-(Me 3 SiN) 2 C 6 //4 protons and phenyl imido ortho protons, 4 H). 13 C{ *H} NMR 

(C 6 D 6 , 20°C): 5 1.1, 2.7, 124.0, 124.2, 124.6, 125.0, 129.2, 135.0, 158.2, 203.4. Anal. 

Calcd for C 2 6H45MoN 3 Si4: C, 51.37; H, 7.46; N, 6.91. Found: C, 51.13; H, 7.00; N, 7.03. 

Reaction of [(Mo(NPh)-n. 2 -(PhCCPh)(o-(Me 3 SiN) 2 C 6 H 4 )] (79) with tert-Butyl 
Isocyanide: Synthesis and Characterization of 82 

An NMR tube containing 79 in C 6 D 6 was charged with 1.0 equiv of Bu'NC. 

Complex 82 formed after several weeks and was characterized by NMR spectroscopy. 





124 

The isolation of complex 82 has not been attempted at this point. For selected NMR data 
see Figure 4-20. 

[Mo(NPh)-ri 2 -PhN=C(H)Ar(0-(Me3SiN) 2 C6H 4 )l (Ar = C 6 H 4 -p-OMe) (83) 

To a green solution of 38 (0.50 g, 1 .05 mmol) in pentane at 20°C was added a 
pentane solution of PhN=C(H)Ar (Ar - C 6 H 4 -/?-OMe) (0.22 g, 1.05 mmol). After stirring 
for 1 2 h, the pentane solution was concentrated in vacuo and cooled, giving crystals of 
83. 'H NMR (C 6 D 6 , 20°C): 5 0.31 (SiMe 3 , 9 H), 0.42 (SiM? 3 , 9 H), 3.33 (OMe), 4.79 (br, 
CH), 7.4-6.1 (ov, m, br, Ph, 18 H). 'H NMR (C 7 D 8 , -90°C): 5 0.33 (SiM> 3 , 9 H), 0.49 
(SiMe 3 , 9 H), 3.26 (OMe), 4.77 (imine CH), 5.0 (d, br, 3 J C -h = 7 Hz, ortho H), 6.1 (d, br, 
3 J C -h = 1 Hz, ortho H), 7.5-6.4 (ov, m, Ph, 16 H). I3 C{'H} NMR (C 6 D 6 , 20°C): 5 1.9 (ov, 
SiMe 3 ), 55.0 (OMe), 62.2 (br, imine C(H)), 113.7 (ov), 119.3, 120.9, 124.0, 124.5, 124.9, 
125.1, 125.5, 125.7, 126.6, 128.8, 129.1, 133.0. 138.4, 153.4, 157.5, one aromatic 
resonance obscured by solvent. HRMS calcd for [M]: 650.1796 m/e. Found (LSIMS): 
650.1737 m/e. I.R.: 1589 cm" 1 . 

[Mo(NPh)-r| 2 -PhN=C(Me)Ph(o-(Me3SiN) 2 C 6 H 4 )](84) 

To a green solution of 38 (0.50 g, 1 .05 mmol) in pentane at 20°C was added a 
pentane solution of PhN=C(H)Ar (Ar = C 6 H 4 -/?-OMe) (0.20 g, 1.05 mmol). After stirring 
for 12 h, the pentane solution was concentrated in vacuo and cooled, giving crystals of 
84. 'H NMR (C 6 D 6 , 20°C): 5 0.34 (SiM? 3 ), 0.41 (SiM? 3 ), 2.34 (br, Me), 7.4-6.4 (ov, m, 
br, Ph, 19 H). 'HNMR (C 7 D 8 , -65°C): 5 0.31 (SiMe 3 ), 0.41 (SiM? 3 ), 2.55 (Me), 5.18 (d, 
br, V c -H = 8 Hz, ortho H), 6.27 (t, 3 J C . H - 8 Hz, para H), 6.35 (d, V C -h = 8 Hz, ortho H), 
6.61 (t, Vc-h - 8 Hz,para H), 7.4-6.2 (ov, m, Ph, 15 H). 13 C{'H} NMR (C 6 D 6 , 20°C): 5 
2.5 (SiMe 3 ), 2.6 (SiM? 3 ), 23.3 (Me), 66.6 (br, imine C), 119.6, 121.0, 123.5, 124.1, 124.9, 



125 



125.3, 125.4, 125.8, 126.2, 126.8, 127.9, 129.3, 129.6, 134.1, 134.5, 148.7, 151.3, 158.1. 
HRMS calcd for [M + H] + : 635.1925 m/e. Found (LSIMS): 635.1854 m/e. I.R.: 1580 



cm" 1 . 



[Mo(NPh)EtNC(H)ArC(H)ArNEt(o-(Me 3 SiN) 2 C 6 H4)] (Ar = C 6 H 4 -/>-OMe) (85) 
To a green solution of 38 (0.50 g, 1.05 mmol) in pentane at 20°C was added a 
pentane solution of EtN=C(H)Ar (Ar = C 6 H 4 -/?-OMe) (0.34 g, 2.10 mmol). After stirring 
for 12 h, the pentane solution was concentrated in vacuo and cooled, giving crystals of 
85. 'H NMR (C 6 D 6 , 20°C): 5 0.28 (SiMe 3 , 9 H), 0.38 (SiM> 3 , 9 H), 0.62 (t, V C -h = 7 Hz, 
Me), 0.89 (t, Vc-h = 7 Hz, Me), 3.26 (d of q, 2 J C . H = 13 Hz, 3 J C -h = 7 Hz, methylene CH), 
3.36 (OMe), 3.38 (OMe), 3.81 (d of q, 2 J C -h = 13 Hz, 3 J C -h - 7 Hz, methylene CH), 4.01 
(d of q, V c -H = 13 Hz, V C -h - 7 Hz, methylene CH), 4.19 (d of q, 2 J C -h - 13 Hz, 3 J C -h = 7 
Hz, methylene CH), 4.65(NC(//)), 4.88 (NC(tf)), 7.6-6.8 (ov, m, Ph, 17 H). ,3 C{'H} 
NMR (C 6 D 6 , 20°C): 8 2.5 (SiM> 3 ), 3.5 (SiM? 3 ), 15.8 (Me), 17.3 (Me), 55.3 (OMe), 55.4 
(OMe), 58.2 (NC(H)), 59.4 (NC(H)), 83.4 (NCH 2 ), 85.7 (NCH 2 ), 114.2, 114.3, 121.6, 
123.2, 123.3, 124.5, 126.7, 127.2, 128.9, 129.1, 129.4, 138.1, 138.8, 146.2, 146.3, 156.9, 
159.5, 159.7. HRMS Calcd for [M + H] + : 766.2878 m/e. Found (LSIMS): 766.2818 m/e. 
[Mo(NPh)BzNC(H)ArC(H)ArNBz(0-(Me 3 SiN) 2 C 6 H 4 )] (Ar = C 6 H 4 -/>-OMe) (86) 
To a green solution of 38 (0.50 g, 1.05 mmol) in pentane at 20°C was added a 
pentane solution of BzN=C(H)Ar (Ar - C 6 H 4 -/?-OMe) (0.47 g, 2.10 mmol). After stirring 
for 12 h, the pentane solution was concentrated in vacuo and cooled, giving crystals of 
86. 'H NMR (C 6 D 6 , 20°C, 500MHz): 5 0.26 (SiM> 3 , 9 H), 0.39 (SiM? 3 , 9 H), 3.35 
(OMe), 3.36 (OMe), 4.50 (NC(//)), 4.51 (d, 2 J C . H = 15 Hz, benzylic proton), 4.64 (d, 2 J C -h 
= 15 Hz, benzylic proton), 4.75 (NC(/f)), 5.45 (d, 2 J C m = 15 Hz, benzylic proton), 5.51 



126 

(d, Vc-h = 15 Hz. benzylic proton), 7.6-6.8 (ov, m, Ph, 27 H). Selected 13 C NMR from 
GHMBC and GHMQC (C 6 D 6 , 20°C, 500 MHz): 8 2.1 (SiMe 3 ), 2.5 (SiM> 3 ), 54.6 
(overlapping OMe), 66.6 (benzylic carbon), 66.8 (benzylic carbon), 80.2 (NC(H)), 81.6 
(NC(H)). HPvMS calcd for [M + H] + : 890.3 1 84 m/e. Found (LSIMS): 890.3 1 57 m/e. 
[(Mo(NPh)(C(Me) 2 CH 2 C(Me) 2 0)(o-(Me 3 SiN) 2 C6H4)](88) 

Acetone (0.037 g, 0.65 mmol) was added to a stirring, green solution of 38 (0.32 g, 
0.65 mmol) in 50 mL of pentane at 20°C. The color of the reaction mixture immediately 
changed from green to purple. The resulting pentane solution was concentrated in vacuo 
and cooled at -30°C for 2 days, affording purple microcrystals that were isolated by 
filtration and dried in vacuo. Additional crops can be crystallized from the mother 
liquors affording 88 in 78% isolated yield. ! H NMR (CDC1 3 , 20°C): 5 0.27 (SiM? 3 , 9 H), 
0.28 (SiM? 3 , 9 H), 0.53 (Me), 0.95 (Me), 1.26 (Me), 1.86 (Me), 2.08 (d, 13 Hz, C(H)), 
2.99 (d. 13 Hz, C(H)), 6.9-7 .4 (aromatic protons, 9 H). 13 C{ ! H} NMR (CDC1 3 , 20°C): 5 
0.54, 3.28, 30.8, 31.4, 34.4, 39.5, 67.8, 78.0, 83.8, 1 19.7, 120.9, 122.0, 125.4, 125.5, 
125.8, 128.7, 139.5, 145.7, 156.7. HRMS calcd for [M + H] + : 554.1926 m/e. Found 
(LSIMS): 554.1948 m/e. 

[(Mo(NPh)(C(H)PhCH 2 C(Me) 2 0)(o-(Me 3 SiN) 2 C 6 H 4 )](89) 

Acetone (0.022 g, 0.38 mmol) was added to a stirring, green solution of 87 (0.20 g, 
0.38 mmol) in 50 mL of pentane at 20°C. The color of the reaction mixture immediately 
changed from green to red. The resulting pentane solution was concentrated in vacuo and 
cooled at -30°C for 2 days, affording red microcrystals that were isolated by filtration and 
dried in vacuo. Additional crops can be crystallized from the mother liquors affording 89 
in 71% isolated yield. ] H NMR (C 6 D 6 , 500MHz, from GHMBC and GHMQC): 5 -0.06 



127 

(SiMe 3 , 9 H), 0.38 (SiM> 3 , 9 H), 0.78 {Me), 1.47 {Me), 2.56 (m, 2 H), 3.16 ("t", 14.5 Hz, 
1 H), 6.85 (t of t, 7.5 Hz, 1.0 Hz, N-Ph imido para proton), 6.91 (toft, 7.5 Hz, 1.0 Hz, 
metallacycle phenyl para proton), 7.02 (d of d of d, 8 Hz, 7 Hz, 1.5 Hz, o-(Me 3 SiN) 2 C 6 #4 
proton), 7.09 (t, 8 Hz, N-Ph imido meta protons), 7.10 (ov, o-(Me 3 SiN) 2 Cy/4 proton), 
7.17 (t, 7.5 Hz, metallacycle phenyl meta protons), 7.30 (d of d, 8 Hz, 1 Hz, metallacycle 
phenyl ortho protons), 7.32 (d of d, 8.5 Hz, 1.0 Hz, o-(Me 3 SiN) 2 Cy/4 proton), 7.40 (d of 
d, o-(Me 3 SiN) 2 C 6 // 4 proton) 7.47 (d of d, 8.5 Hz, 1.0 Hz, N-Ph imido ortho protons). 13 C 
(C 6 D 6 , 500MHz, from GHMBC and GHMQC): 5-0.1, 1.0, 28.0, 32.4, 56.2, 76.7, 86.4, 
121.2, 123.6, 123.7, 123.8, 124.3, 125.6, 127.6, 128.0, 128.2, 128.9, 133.2, 140.3, 154.0, 
156.2. HRMScalcd for [M] + : 601.1844 m/e. Found (LSIMS): 601.1869. 
[(Mo(NPh)(C(H)PhCH 2 C(Et)2O)(o-(Me 3 SiN)2C 6 H4)](90) 

To a C 6 D 6 solution (0.7 mL) of 87 (0.050 g, 0.092 mmol) in a vial was added 
3-pentanone (0.0080 g, 0.092 mmol). The contents of the vial were then transferred to an 
NMR tube. The reaction was monitored by 'H NMR for 1 week. Metallacycle 90 was 
never isolated, and only *H NMR has been collected. *H NMR (C 6 D 6 , 20°C): 5 -0.05 
{SiMe 3 , 9 H), 0.42 (SiM? 3 , 9 H), 0.62 (t, 7.5 Hz, Me), 0.8 (ov, m, methylene C{H), 2 
H),1.01 (t, 7.5 Hz, Me), 1.15 (m, methylene C{H)), 1.78 (m, methylene C{H)), 2.64 (m, 
metallacycle protons, 2 H), 3.11 ("t", 13 Hz, metallacycle proton), 6.8-7.4 (ov, m, 14 H). 
[(Mo(NPh)(C(H)PhCH 2 C(CH 2 ) 5 0)(o-(Me 3 SiN) 2 C 6 H 4 )](91) 

Cyclohexanone (0.036 g, 0.37 mmol) was added to a stirring, toluene solution of 87 
(0.20 g, 0.37 mmol) at 20°C. Metallacycle 91 was isolated in 75% yield by 
crystallization from pentane at -30°C followed by filtration. ] H NMR (CDC1 3 , 20°C): § 
-0.15 (SiMe 3 , 9 H), 0.32 (SiM> 3 , 9 H), 0.8-1.8 (ov, m, cyclohexyl protons), 2.37 (d of d, 






128 

14 Hz, 5 Hz, metallacycle C(H)), 2.53 (d of d, 13 Hz, 5 Hz, metallacycle C(H)), 2.85 ("t", 
13.5 Hz, metallacycle C(H)), 6.9-7.4 (ov, m, aromatic protons). 13 C{'H} NMR (CDC1 3 , 
20°C): § 0.7, 1.2, 23.2, 23.8, 26.4, 37.2, 41.9, 54.8, 75.4, 84.9, 121.4, 123.5, 123.6, 123.9, 
124.5, 125.6, 127.4, 127.8, 128.2, 128.9, 133.1, 140.1, 154.5, 156.2. HRMS calcd for [M 
+ H] + : 641.2262 m/e. Found (LSIMS): 641.2209 m/e. 
[(Mo(NPh)(C(Me) 2 CH 2 C(H)(C 6 H 4 - />-OMe)O)(0-(Me 3 SiN) 2 C 6 H 4 )] (92) 

To a stirring solution of 38 (0.810 g, 1.64 mmol) in pentane at 20°C was added 
/?-anisaldehyde (0.223 g, 1.64 mmol). Compound 92 was isolated in 78% yield by 
crystallization from pentane at -30°C followed by filtration. 'H NMR (CDC1 3 , 20°C): S 
0.31 (SiMe 3 , 9 H), 0.40 (SiA& 3 , 9 H), 0.64 (Me), 1.98 (Me), 2.26 (d of d, 13 Hz, 5 Hz, 
metallacycle C(H)), 2.87 ("t", 1 1 Hz, metallacycle C(H)), 3.77 (OMe), 5.05 (d of d, 1 1 
Hz, 6 Hz, metallacycle C(H)), 6.78 (d, 9 Hz, aromatic proton, 2 H), 7.0-7.4 (aromatic 
protons, 1 1 H). I3 C{'H} NMR (CDCI3, 20°C): 5 0.8, 3.0, 28.0, 38.4, 55.4, 63.1, 77.7, 
83.3, 113.5, 121.0, 122.5, 122.7, 125.0, 125.6, 126.5, 126.7, 128.7, 137.8, 140.5, 143.4, 
156.5, 158.2. HRMS calcd for [M + H] + : 632.2050 m/e. Found (LSIMS): 632.2031 m/e. 
Reactivity of Arene Complexes with Acetone 

A representative reaction: To a C 6 D 6 solution of appropriate arene complex was 
added 1.0 equiv of acetone using a uL syringe. The proposed products, 93 and 94, and 
supporting spectroscopic data are discussed in the text. 


















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BIOGRAPHICAL SKETCH 

Thomas Michel Cameron was born July 29, 1975 in Smiths Falls Ontario, to 
parents Lucile and Gerald Cameron. His younger sister, Dominique, was born two years 
later, and the Cameron family of four moved from Perth (Ontario) to Ottawa (Ontario), 
then Puerto Rico and finally to Montreal. Tom attended high school and college in the 
Montreal area. He left Montreal to attend school at Mt. Allison University in Sackville, 
New Brunswick, from which he graduated in 1 997. 

Tom began his laboratory experience at Mount Allison under the direction of 
Professor Steven A. Westcott. After graduating, Tom went to Los Alamos National Labs 
for one year and gained valuable lab experience while working with Dr. R. Tom Baker. 
Tom left Los Alamos to pursue a graduate degree at the University of Florida under the 
direction of advisor Professor James M. Boncella. The work in this manuscript is a fairly 
complete record of what has been accomplished during his 4.5 years at UF. Upon his 
graduation, Tom will return to Los Alamos where his wife, Melissa, is waiting for him. 



139 



I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 




r tsni*»% IV/t^sM*/ 






imes M. Boncella, Chair 
Professor of Chemistry 



I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 




VL 



Daniel R. Talham 
Professor of Chemistry 

I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 




u-V ^CA^\^^^ 



Kirk S. Schanze 
Professor of Chemistry 

I certify that I have read this study and that in my opinion it conform* 
acceptable standards of scholarly presentation and is fully adequate, in so<5pe and quality, 
as a dissertation for the degree of Doctor of Philosophj 





Michael J. Scott 

Associate Professor of Chemistry 



I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 




Eugene Goldberg 
Professor of Materials Scft 
Engineering 



This dissertation was submitted to the Department of Chemistry in the College of 
Liberal Arts and Sciences and to the Graduate School and was accepted as partial 
fulfillment of the requirements for the degree of Doctor of Philosophy. 

December 2002 



Dean, Graduate School 
















,CK* 



UNIVERSITY OF FLORIDA 

Illllllll 

3 1262 08555 3302